292 



MOLLUSCA FROM THE CRAG, 



Gastroch^ena,* Spongier, 1783. 



Uperotus (part). Guettard, 1774. 

 Ch.ena (part). Retzius, 1788. 

 Mya (sp.) Pennant. 

 Pholas (sp.) Chemn. Poll. 



Fistulana (sp.) Brug. 1/92. 

 Trapezium (sp.) Megerle, 1811. 

 Gastrochina. Swains. 1840. 



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 of 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. Gastroch/ENA dubia, Pennant. Tab. XXX, fig. 11, a — d. 



Mya dubia. Penn. Brit. Zool., vol. iv, p. 82, pi. 44, fig. 19, 1777. 



Chama parva. Da Costa. Brit. Conch., p. 234, 1778. 



Mya pholadia. Mont. Test. Brit., p. 28, 1803; and Supp., p. 20, 1808. 



Pholas faba. Pulteney. Hntchin's Hist. Dorset, p. 27. 



Gastroch/ENA modiolina. Lamk. Hist, des An. s. Vert., torn, v, p. 447, 1818. 



— — Forb. and Hani. Hist, of Brit. Moll., p. 132, pi. 2, figs. 5—8, 



pi. F, fig. 5. 



— pholadia. Turt. Brit. Biv., p. 18, pi. 2, figs. 8, 9, 1822. 



— — Lukis. Mag. Nat. Hist., vol. vi, p. 404, fig. 52, a — g. 



— cuneiforjiis. Phil. En. Moll. Sic, vol. i, p. 2, 1836. 



— polii. Id. „ ,, „ vol. ii, p. 3, 1844. 



— iitans. Flem. Hist. Brit. An., p. 458, 1828. 



fulva. Leach, MS. 1818. Ann. Mag. Nat. Hist., vol. xx, p. 272. 



— dubia. Uesh. Traite Elem. Conch., pi. 2, figs. 4, 5. 

 Poliana. Phil. In Wiegman Archiv. Nat., pi. 7, fig. 1, 1845. 



Etym. yaarnp, ventral, and xatvw, to gape. 



