458 MOLLUSCA. PHOLAD^. Gastroch^ena. 



cand. Linn. Syst. i. 1111. Mont. Test. Brit. 24. Turt. Biv. Brit. 10. 

 In clay rocks. 

 Length 1 inch, breadth 2^ inches ; white, fragile, and rounded at both ends ; 

 concentrical ridge divided into longitudinal rows of short prickles. 



EXTINCT SPECIES. 



1. p. cylindricus Transversely elongated, nearly cylindrical; anterior 



side muricated, pointed, with a sinus in the edge ; beaks concealed by a re- 

 flection of the edges of the back. — Sower. Min. Conch, t. cxcviii — In Crag. 



Gen. CXXXIV. GASTROCH^ENA. — Shell transverse, 

 equivalve, inequilateral, with a large oblique anteal open- 

 ing ; hinge with a single transverse laminar tooth in each 

 valve ; ligament external, rather remote from the beaks. 



560. G. Mans. — Beaks nearly terminal, rather prominent, 



Mya dubia, Penn. Brit. Zool. iv. 82. t. xliv. Mont. Test. Brit. 28 — 



G. pholadia, Turt. Biv. Brit. 18. t. ii. f. 8, 9.— In Limestone, near low 



water-mark, England. 



Length half an inch, breadth an inch ; finely striated concentrically ; from 



the beak to the ligament the margin is nearly straight, then rounded retral- 



ly ; the oblique anteal truncation extended beyond the middle of the ventral 



margin ; a narrow border in front of the beaks, where the valves are in con- 



tact. 



EXTINCT SPECIES. 



1. G. tortuosa. — Obliquely lanceolate and twisted.— .Sowjer. Min. Conch. 

 t. Dxxvi, f. 1. — In Sandstone, Robin Hood's Bay, near Scarborough. 



2. G. contorta Sheath clavate, bent nearly at a right angle ; valves ovate, 



elongated ; marked with very slender striae ; the sinus between them mde, 

 oval, pointed Sower. Min. Conch, t. Dxxvi. f, 2 — London Clay. 



SOLENID^. 



Gen. CXXXV. SOLEN.— Shell transversely produced, the 

 dorsal and ventral margins nearly parallel ; the extremities 

 gaping ; teeth projecting ; cuticle strong. 



* Hinge terminal, the anteal extremity truncated. 



561. S. Vagina. — A single tooth in each valve, with a stric- 

 ture across the anteal extremity of the shell. 



List. Conch, t. mIvI. Linn. Syst. i. 1113. Penn. Brit. Zool. iv. 83. 



Mont. Test. Brit. 48. Turt. Biv. Brit. 79 — English and Irish coasts. 



Length 1 inch, breadth 5 inches ; subcylindrical, becoming more compressed 



towards the truncated retral extremity ; the anteal extremity is slightly 



