458 
MOLLUSC A. PHOLADiK. GASTRocHiENA. 
cand. Linn. Syst. i. 1111. Mont. Test. Brit. 24. Turt. Biv. Brit. 10. 
In clay rocks. 
Length 1 inch, breadth 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. GASTROCHJENA. — Shell transverse, 
equivalve, inequilateral, with a large oblique anieal open- 
ing ; hinge with a single transverse laminar tooth in each 
valve ; ligament external, rather remote from the beaks. 
560. G. Ilians. — 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. — 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. Mlvi. Linn. Syst. i. 1113. Penn. Brit. Zool. iv. 83. 
Mont. Test. Brit. 48. Turt. Biv. Brit. 79 — English anij Irish coasts. 
Length 1 inch, breadth 5 inches ; subcylindrical, becoming more compressed 
towards the truncated retral extremity ; the anteal extremity is slightly 
