212 MOLLUSCA. LAMELLIBRANCHIA 



Mytilus. Shell thin, equivalve, very inequilateral, elongated, 

 sub-triangular, posterior border rounded. Umbones sharp, 

 terminal, anterior. Teeth small or absent. Ligament linear, 

 marginal, sub-internal. Anterior adductor impression small, placed 

 near the umbo ; posterior large ; pallial line simple. Trias to 

 present day. Ex. M. edulis, Pliocene to present day. 



Modiola. Shell similar to Mytilus, but oblong, inflated in 

 front. Umbones obtuse, anterior, but not terminal. Devonian to 

 present day. Ex. M. modiola, Recent ; M. imbricata, Inferior 

 Oolite. 



Lithodomus ( = Lithophagus). Shell similar to Modiola; 

 sub-cylindrical, rounded in front, wedge-like behind. Lithodomus 

 bores into limestone, etc. Carboniferous to present day. Ex. L. 

 inclusus, Inferior Oolite to Corallian. 



Modiolopsis. Shell thin, smooth, elongate, very inequilateral, 

 anterior part small, posterior part enlarged. Umbones nearly 

 terminal, close together ; a depression crosses the valves obliquely 

 from the umbo. No teeth. Anterior adductor impression deep ; 

 posterior adductor large, faintly marked. Ordovician and Silurian. 

 Ex. M. modiolaris, Ordovician. 



Myoconcha. Similar to Modiolopsis, but usually with a long 

 cardinal and a long slender posterior lateral tooth in the right valve. 

 Carboniferous to Chalk. Ex. M. crassa, Inferior Oolite ; M. cretacea, 

 Chalk. 



Hippopodium. Shell very thick, very convex, oblong ; 

 surface with lines of growth. Umbones large, anterior. Hinge 

 thick, with one oblique tooth which may disappear in old specimens. 

 Adductor impressions deep. Pallial line simple. Lias to Great 

 Oolite. Ex. H. ponderosum, Lower and Middle Lias. 



Myalina. Shell thick, trigonal, oblique, very inequilateral, 

 with pointed umbones at the anterior extremity. Anterior marginal 

 part of valves sharply bent. Posterior part compressed, wing-like. 

 Hinge-line straight, long. Hinge-margin broad with longitudinal 

 striations. Anterior adductor near the ventral edge of the anterior 

 end of the hinge-plate. Posterior adductor large, oval. Pallial line 

 simple. Surface with growth-lines, often lamellar. Silurian to 

 Permian ; common in Carboniferous. Ex. M. verneuili, Carboni- 

 ferous. 



