MOLLUSCA. LAMELLIBRANCHIA 227 



Pleuromya. Shell transversely elongated, anterior side short, 

 posterior long and generally compressed, sometimes gaping ; surface 

 with concentric folds. Hinge without teeth, but with a thin pro- 

 jecting lamina at the margin. Ligament partly external. Adductor 

 impressions faintly marked ; pallial sinus deep. Trias to Cre- 

 taceous. Ex. P. do?iacina, Corallian and Kirneridgian. 



Gresslya. Shell oval, elongate, very inequilateral, smooth or 

 with concentric furrows ; anterior side high and inflated, posterior 

 side narrowing and somewhat compressed. Umbones anterior, 

 close together; lunule sometimes well marked. Right valve a little 

 higher and larger than the left. Adductor impressions shallow ; 

 pallial sinus deep. Behind the umbo of the right valve is a tooth- 

 like projection and an internal plate — the latter appears as a furrow 

 in casts of the shell. Jurassic. Ex. G. gregaria, G. abducta, 

 Inferior Oolite. 



Ceromya. Shell heart-shaped, inflated, inequilateral, finely 

 granular, with concentric grooves. Left valve not quite so convex 

 as the right. Anterior side short, posterior longer and compressed. 

 Umbones prominent, anterior, curved forward. Hinge thickened, 

 with a ridge behind the umbones ; teeth absent. Pallial line 

 sinuous. Jurassic. Ex. C. concentrica, Inferior Oolite to Corn- 

 brash. 



Pholadomya. Shell thin, translucent, oblong or oval, 

 ventricose, equivalve, gaping posteriorly and sometimes anteriorly. 

 Anterior side short and rounded. * Surface with radiating ribs 

 crossed by concentric striae. Umbones prominent. Ligament ex- 

 ternal. Hinge without teeth or with a small transverse tubercle. 

 Adductor impressions very faint. Pallial sinus deep. Lias to 

 present day. Ex. P. margaritacea, London Clay. 



Homomya. Similar to Pholadomya. Without radial ribs ; 

 surface smooth or ornamented with fine granules. Trias to Creta- 

 ceous. Ex. H. gibbosa, Inferior and Great Oolite. 



Goniomya. Similar to the last two, but with V-shaped ribs 

 pointing ventrally. Lias to Cretaceous. Ex. G. literata, Inferior 

 Oolite to Corallian. 



Thracia. Shell thin, oblong, compressed, attenuated and 

 gaping posteriorly ; surface smooth or concentrically striated. L"m- 



15-2 



