292 MOLLUSCA FROM THE CRAG. 
GASTROCHENA,* Spengler, 1783. 
Urrrotus (part). Guettard, 1774. | FistuLana (sp.) Brug. 1792. 
Cuana (part). Retzius, 1788. | TRAPEZIUM (sp.) Megerle, 1811. 
Mya (sp.) Pennant. GastTROcHINA. Swains. 1840. 
Puoxas (sp.) Chemn. Poli. | 
Generic Character. Shell equivalve, inequilateral, oval or wedge-shaped, with a 
large ventral opening ; hinge linear; a single laminated obscure tooth in each valve. 
Ligament external. 
Tube calcareous, pyriform ; posterior opening narrow, with a bipartite or divided 
aperture. ; 
Animal cuneiform, siphons united to their extremities, orifices fringed, mantle 
closed, with the exception of an opening for a small, finger-shaped, curved, and 
pointed foot, sometimes furnished with a delicate byssus. 
Animals of this genus are generally enclosed in the thick shelly substance a some 
mollusc, or in the centre of a mass of coral, their excavations being produced by an 
operation of the animal itself. The mode by which this is effected is not yet satisfac- 
torily determined, a difference of opinion still existing as to whether it is caused by 
the action of a solvent, or by the mechanical operation of surface abrasion. 
Some specimens do not attempt to excavate, but merely to enclose themselves in 
their own flask-like cases, to which occasionally foreign materials are added. 
Species of this genus have been found in the Oolite Formations, and in the Older 
Tertiaries, but nowhere in any abundance. 
1. GastROCHANA DUBIA, Pennant. ‘Tab. XXX, fig. 11, a—d. 
Mya pusia. Penn. Brit. Zool., vol. iv, p. 82, pl. 44, fig. 19, 
Cuama paRVA. Da Costa. Brit. Conch., p. 234, 1778. 
Mya puotapta. Mont. ‘Test. Brit., p. 28, 1803; and Supp., p. 20, 1808. 
PHouas FABA. Pulteney. Hutchin’s Hist. Dorset, p. 27. 
GASTROCHHZNA MODIOLINA. Lamk. Hist. des An. s. Vert., tom. v, p. 447, 1818. 
— = Forb. and Hanl. Hist, of Brit. Moll., p. 132, pl. 2, figs. 5— 
pl. F, fig. 5 
os PHOLADIA. Turt. Brit. Biv., p. 18, pl. 2, figs. 8, 9, 1822. 
— — Iukis. Mag. Nat. Hist., vol. vi, p. 404, fig. 52, a—g. 
—_ CUNEIFORMIS. Phil. Wn. Moll. Sic., vol. i, p. 2, 1836. 
— POLIL. Id. een op voll yoy Sh WEEE 
— HIANS. J'lem. Hist. Brit. An., p. 458, 1828. 
— FULVA. Leach, MS. 1818. Ann. Mag. Nat. Hist., vol. xx, p. 272. 
— puBIa. Desh. Traité Elém. Conch., pl. 2, figs. 4, 5. 
_ Poriana. Phil. In Wiegman Archiv. Nat., pl. 7, fig. 1, 1845. 
(eo) 
* Etym. yaornp, ventral, and xatvw, to gape. 
