''Hemisurcula'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

sea snail Sea snail is a common name for slow-moving marine gastropod molluscs, usually with visible external shells, such as whelk or abalone. They share the taxonomic class Gastropoda with slugs, which are distinguished from snails primarily by the ...

s, marine

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

s in the

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Clavatulidae Clavatulidae is a taxonomic family of sea snails, marine gastropod mollusks in the superfamily Conoidea. The family is not well differentiated morphologically. Clavatulidae was raised, based on cladistic analysis, from subfamily to the family ...

.Marshall, B. (2015). Hemisurcula Casey, 1904 †. In: MolluscaBase (2015). Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=829411 on 2016-06-11

Description

(Original description) In this genus the shell is fusiform, with the protoconch conoidal, multispiral and closely coiled. The nepionic (= relating to the stage of development succeeding the embryonic) spire whorls alone are costate and have also an elevated collar below the suture. The more recent whorls become devoid of lyrae or costae, though having throughout densely close-set and subequal microscopic striae, except the body whorl abruptly below the posterior end of the

aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...

, which is obliquely and rather coarsely lyrate. The

siphonal canal The siphonal canal is an anatomical feature of the shells of certain groups of sea snails within the clade Neogastropoda. Some sea marine gastropods have a soft tubular anterior extension of the mantle called a siphon through which water is ...

is moderate, straight, and, together with the aperture, forms about half the length of the shell. The sinus is broadly rounded and median in position on the spire whorls. The

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is simple. The type of this genus is ''PIeurotoma silicata'', of Aldrich, a very remarkable and isolated species occurring in the Lignitic

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

of the Gregg's Landing beds of

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. The beaded subsutural collar, subjacent depression and swollen and finely ribbed lower parts of the two

whorl A whorl ( or ) is an individual circle, oval, volution or equivalent in a whorled pattern, which consists of a spiral or multiple concentric objects (including circles, ovals and arcs). Whorls in nature File:Photograph and axial plane floral ...

s immediately below the

protoconch A protoconch (meaning first or earliest or original shell) is an embryonic or larval shell which occurs in some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod. In older texts it is also called ...

are lost completely on the larger whorls, though the subsutural collar can be feebly traced as a slightly tumid line gradually descending further below the suture with the growth of the shell. Besides ''silicata'', the genus will include the much stouter ''PIeurotoma roscoei'' Harris, from the same horizon.

Species

* † '' Hemisurcula oborni'' (Marwick, 1960) * † '' Hemisurcula perexilis'' Aldrich 1886 * † '' Hemisurcula terus'' Garvie, 2013 Garvie, Christopher L. "Additions to the molluscan macrofauna of the Reklaw Formation (Eocene: Lower Claibornian) and two new taxa from the middle Claibornian in Texas." Bulletins of American Paleontology 384 (2013).

References

* Maxwell, P.A. (2009). Cenozoic Mollusca. pp 232–254 in Gordon, D.P. (ed.) ''New Zealand inventory of biodiversity. Volume one. Kingdom Animalia: Radiata, Lophotrochozoa, Deuterostomia.'' Canterbury University Press, Christchurch {{Taxonbar, from=Q24898764