458 MOLLUSCA. PHOLADiE. Gastrocilena. 



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. exeviii — In Crag. 



Gen. CXXXIV. GASTROCILENA. — 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. tortuoscu — Obliquely lanceolate and twisted. — Sower. Min. Conch, 

 t. Dxxvi, f. 1. — In Sandstone, Bobin 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 wide, 

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



SOLENIDiE. 



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. ailvi. 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 



