MOLLUSCA. LAMELUBRANCHIA 219 



Megalodon. Shell thick, equi valve, smooth or with con- 

 centric lines, convex, inequilateral, oval or rounded triangular. 

 Umbones prominent, curved forward. Ligament external, long. 

 Hinge-plate very large and thick ; teeth thick ; in the right valve 

 two cardinals separated by a pit ; in the left valve one cardinal 

 under the umbo and a small anterior cardinal ; no laterals. 

 Anterior adductor impression small, semilunar ; posterior adductor 

 long, shallow, on a ridge extending from the hinge to the posterior 

 border. Devonian to Jurassic. Ex. M. cucullatus, Devonian. 

 Pachyrisma (Trias and Jurassic) is allied to Megalodon. 



5. Hinge heterodont ; hinge-plate usually well-de- 

 veloped. Two nearly equal adductors. Ligament behind 

 the umbones. Siphons usually well-developed. Shell 

 without a pearly layer. 



(a) Pallial line usually simple. 



Cyprina. Shell orbicular or oval, convex, with concentric 

 striae and a thick periostracum. Umbones prominent, incurved. 

 Ligament external, prominent. Lunule seldom present. Right 

 valve with a small anterior cardinal below a triangular median 

 cardinal, an oblique bifid posterior cardinal, and a posterior lateral. 

 Left valve with a small anterior cardinal, a vertical median cardinal, 

 and a long oblique posterior cardinal. Adductor impressions oval. 

 Pallial line entire. Margins of valves smooth. Lias to present day. 

 Ex. C. islandica, Coralline Crag to present day. 



Isocardia. Similar to Cyprina. Umbones inflated, curved 

 anteriorly or spirally inrolled. In each valve two nearly parallel 

 cardinal teeth and one posterior lateral. Jurassic to present day. 

 Ex. /. cor, Coralline Crag to present day. 



Astarte. Shell thick, inequilateral, more or less trigonal or 

 sub-orbicular, compressed, closed. Surface usually with concentric 

 furrows or striae. A thick periostracum is present. Umbones 

 prominent. Lunule distinct. Escutcheon elongated. Ligament 

 external. Two cardinal teeth in each valve, lateral teeth rudi- 

 mentary. Adductor impressions strongly marked ; above the 

 anterior one is a pedal impression. Pallial line simple. Trias to 

 present day. Ex. A. omalii, Coralline Crag. 



