|Apertural view of a shell of Pyramidella acus|
J. E. Gray, 1840
Pyramidellidae, common name the pyram family, or pyramid shells, is a voluminous taxonomic family of mostly small and minute ectoparasitic sea snails, marine heterobranch gastropod molluscs. The great majority of species of pyrams are micromolluscs.
The pyram family is distributed worldwide with more than 6,000 named species in more than 350 nominal genera and subgenera.
This family of micromollusks has been little studied and the phylogenetic relationships within the family are not well worked out. There is a absence of a general consensus which species belong to a specific genus or subgenus, contributing to much confusion. Schander (1999) names more than 300 supraspecific names.  As there has been no serious generic revision of the genera worldwide, generic polyphyly can be expected to be rampant throughout the family. However, the family itself is deemed monophyletic. 
The family is currently divided into 11 subfamilies (Ponder & Lindberg 1997). An alternative interpretation is that the family Pyramidellidae is but one of six families within the superfamily Pyramidelloidea (Schander, van Aartsen & Corgan 1999). Many species are rare or infrequently recorded.
Subfamilies included within the family Pyramidellidae vary according to the taxonomy consulted. The currently leading taxonomy is the taxonomy of Bouchet & Rocroi (2005).
- Chrysallidinae Saurin, 1958
- Cingulininae Saurin, 1959
- Cyclostremellinae Moore, 1966
- Eulimellinae Saurin, 1958
- Odostomellinae Saurin, 1959
- Odostomiinae Pelseneer, 1928
- Pyramidellinae J.E. Gray, 1840
- Sayellinae Wise, 1996
- Syrnolinae Saurin, 1958
- Tiberiinae Saurin, 1958
- Turbonillinae Bronn, 1849
- Superfamily Pyramidelloidea Gray, 1840
- Family Amathinidae Ponder, 1987
- Family Ebalidae Warén, 1994 - synonym: Anisocyclidae van Aartsen, 1995
- Family Odostomiidae Pelseneer, 1928
- Family Pyramidellidae J. E. Gray, 1840
- Family Syrnolidae Saurin, 1958
- Family Turbonillidae Bronn, 1849
- Subfamily Pyramidellinae Gray, 1840
- Subfamily Odostomiinae Pelseneer, 1928
- Tribe Odostomiini Pelseneer, 1928 - synonyms: Ptychostomonidae Locard, 1886; Liostomiini Schander, Halanych, Dahlgren & Sundberg, 2003 (n.a.)
- Tribe Chrysallidini Saurin, 1958 - formerly subfamily Chrysallidinae, synonyms: Menesthinae Saurin, 1958; Pyrgulininae Saurin, 1959
- Tribe Cyclostremellini D. R. Moore, 1966 - formerly subfamily Cyclostremellinae
- Tribe Odostomellini Saurin, 1959 - formerly subfamily Odostomellinae
- Subfamily Syrnolinae Saurin, 1958 - formerly subfamily Syrnolinae
- Tribe Syrnolini Saurin, 1958
- Tribe Tiberiini Saurin, 1958 - formerly subfamily Tiberiinae
- Subfamily Turbonillinae Bronn, 1849
- Pyramidellidae incertae sedis
The following genera are currently difficult to place within existing subtaxa of the Pyramidellidae.
- Charilda Iredale, 1929
- Contraxiala Laseron, 1956
- Cossmannica Dall & Bartsch, 1904
- Eulimotibera Nomura, 1939
- Morrisonetta Brandt, 1968
- Peristichia Dall, 1889
- Ulfa Dall & Bartsch, 1904
- Vagna Dall & Bartsch, 1904
The following genera have become synonyms (but some species in this genera have not yet been reassigned) :
- Actaeopyramis P. Fischer, 1885: synonym of Monotigma G.B. Sowerby II, 1839
- Amaura Møller, 1842: synonym of Aartsenia Warén, 1991
- Auriculina Gray, 1847 accepted as Ondina de Folin, 1870
- Besla Dall & Bartsch, 1904 accepted as Parthenina Bucquoy, Dautzenberg & Dollfus, 1883
- Brachystomia Monterosato, 1884: synonym of Odostomia Fleming, 1813
- Chemnitzia d'Orbigny, 1839: synonym of Turbonilla Risso, 1826
- Elusa A. Adams, 1861: synonym of Tropaeas Dall & Bartsch, 1904
- Folinella Dall & Bartsch, 1904: synonym of Chrysallida Carpenter, 1856
- Iole A. Adams, 1860: synonym of Iolaea A. Adams, 1867
- Iolina Baily, 1948: synonym of Iolaea A. Adams, 1867
- Ividella Dall & Bartsch, 1909 accepted as Chrysallida Carpenter, 1856
- Jordaniella Chaster, 1898: synonym of Chrysallida Carpenter, 1856
- Lonchaeus : synonym of Longchaeus Mörch, 1875
- Moerchiella Thiele, 1924 : synonym of Moerchia A. Adams, 1860
- Moerchinella Thiele, 1931 accepted as Moerchia A. Adams, 1860
- Monoptygma: synonym of Monotigma G.B. Sowerby II, 1839
- Monotygma G.B. Sowerby II, 1839: synonym of Monotigma G.B. Sowerby II, 1839
- Noemia de Folin, 1870: synonym of Noemiamea de Folin, 1886
- Obeliscus Gray, 1847: synonym of Pyramidella Lamarck, 1799
- Obex Laws, 1940 accepted as Obexomia Laws, 1941
- Odontostoma Philippi, 1853: synonym of Odostomia Fleming, 1813
- Odontostomia G.B. Sowerby I, 1839: synonym of Odostomia Fleming, 1813
- Parthenia Lowe, 1840: synonym of Parthenina Bucquoy, Dautzenberg & Dollfus, 1883
- Parthenina Bucquoy, Dautzenberg & Dollfus, 1883: synonym of Chrysallida Carpenter, 1856
- Partulida Schaufuss, 1869: synonym of Chrysallida Carpenter, 1856
- Planipyrgiscus [sic] accepted as Planpyrgiscus Laws, 1937
- Plotia Röding, 1798: synonym of Pyramidella Lamarck, 1799
- Ptychostomon Locard, 1886 accepted as Odostomia Fleming, 1813
- Pyrgulina A. Adams, 1864: synonym of Chrysallida Carpenter, 1856
- Raoulostraca Oliver, 1915 accepted as Eulimella Forbes & M'Andrew, 1846
- Tragula Monterosato, 1884: synonym of Chrysallida Carpenter, 1856
- Tropeas [sic] accepted as Tropaeas Dall & Bartsch, 1904
The length of the slender, elongated (turreted or conical) shells varies between 0.5 mm and 3.5 cm, but most species in the family have shells which are smaller than 13 mm.
The texture of these shells is smooth or sculptured in various forms such as ribs and spirals. Their color is mostly white, cream or yellowish, sometimes with red or brown lines.
The shell of these snails has a blunt, heterostrophic (i.e. whorls appear to be coiled in the opposite direction to those of the teleoconch) protoconch, which is often pointed sideways or wrapped up. The teleoconch is dextrally coiled, but the larval shells are sinistral. This results in a sinistrally coiled protoconch.
The operculum is ovoid and paucispiral, with the apex anterior, a thread-like arcuate ridge on the proximal side, the inner margin notched in harmony with the plaits of the pillar when prominent.
The Pyramidellidae are ectoparasites, feeding mainly on other molluscs and on annelid worms, but some are known to feed on peanut worms and crustaceans. A few species in the family Pyramidellidae, such as Otopleura mitralis, are symbiotic with sea anemones, such as Neoaiptasia morbilla 
They do not have a radula. Instead their long proboscis is used to pierce the skin of its prey and suck up its fluids and soft tissues. The eyes on the grooved tentacles are situated toward the base of the tentacles. These tentacles have a concave surface. Between the head and the propodium (the foremost division of the foot), a lobed process called the mentum (= thin projection) is visible. This mentum is slightly indented in midline.
- Gray, J. E. (1840). Synopsis of the contents of the British Museum, 42nd edition. London. p. 370 pp.
- Schander C.; Halanych K.M.; Dahlgren T.; Sundberg P (May 2003). "Test of the monophyly of Odostomiinae and Turbonilliinae (Gastropoda, Heterobranchia, Pyramidellidae) based on 16S mtDNA sequences". Zoologica Scripta 32 (3): 243–254. doi:10.1046/j.1463-6409.2003.00112.x.
- Schander C., Hori S. & Lundberg J. (1999), Anatomy, phylogeny and biology of Odostomella and Herviera, with the description of a new species of Odostomella (Mollusca, Heterostropha, Pyramidellidae).Ophelia 51 (1): 39-76
- Angela Dinapoli, Carmen Zinssmeister and Annette Klussmann-Kolb (2010), New insights into the phylogeny of the Pyramidellidae (Gastropoda), Journal of Molluscan Studies Volume 77, Issue 1 Pp. 1-7
- Ponder, W.F. & Lindberg, D. R. (1997). "Towards a phylogeny of gastropod molluscs: an analysis using morphological characters". Zoological Journal of the Linnean Society 119 (2): 88–265. doi:10.1111/j.1096-3642.1997.tb00137.x.
- Schander C., van Aartsen J. J. & Corgan, J. C. (1999). "Families and genera of the Pyramidelloidea (Mollusca: Gastropoda)". Bollettino Malacologico 34 (9–12): 145–166.
- Bouchet, P. & Rocroi, J.-P. (2005). "Classification and Nomenclator of Gastropod Families". Malacologia 47 (1–2): 1–397.
- Dinapoli, Angela; Carmen Zinnsmeister & Annette Klussmann-Kolb (10). "New insights into the phylogeny of the Pyramidellidae (Gastropoda)". Journal of Molluscan Studies 77 (1): 1–7. doi:10.1093/mollus/eyq027. Retrieved 2011-10-25.
- WoRMS (2010). Peristichia. In: Bouchet, P.; Gofas, S.; Rosenberg, G. (2010) World Marine Mollusca database. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=415795 on 2011-03-15
- J.J. van Aartsen, E. Gittenberger & J. Goud, Pyramidellidae (Mollusca, Gastropoda, Heterobranchia) collected during the Dutch CANCAP and MAURITANIA expeditions in the south-eastern part of the North Atlantic Ocean (part 2)
- Sneli, J.-A. (1972). "Odostomia turrita found on Hommarus gammarus". Nautilus 86 (1): 23–24.
- Robertson, R.; Mau-Lastovicka, Terry (1979). "The ectoparasitism of Boonea and Fargoa (Gastropoda: Pyramidellidae)". Biological Bulletin (Marine Biological Laboratory) 157 (2): 320–333. doi:10.2307/1541058. JSTOR 1541058.
- Roger H. Goodwill et. al. (2009), A sea anemone symbiotic with gastropods of eight species in the Mariana Islands; Micronesica 41(1): 117–130, 2009
- Høisæter, T. (1965). "Spermatophores in Chrysallida obtusa (Brown) (Opisthobranchia, Pyramidellidae)". Sarsia 18: 63–68.
- Robertson, R. (1967). "The life history of Odostomia bisuturalis and Odostomia spermatophores (Gastropoda: Pyramidellidae)". American Philosophical Society Yearbook 1967: 368–370.
- Schander C., Hori, S., Lundberg, J. (1999). "Anatomy, Phylogeny and biology of Odostomella and Herviera, with the description of a new species of Odostomella (Mollusca, Heterostropha, Pyramidellidae)". Ophelia 51 (1): 39–76.
- Dinapoli A., Zinssmeister C. & Klussmann-Kolb A. (2010). "New insights into the phylogeny of the Pyramidellidae (Gastropoda)". Journal of Molluscan Studies doi:10.1093/mollus/eyq027.
- Hayward P. J. &, J.S. Ryland (1996). Handbook of the Marine Fauna of North-West Europe. Oxford University Press. p. 546. ISBN 0-19-854055-8.
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