416 MOLLUSCA. UNIONID^. Unio. 



Gen. CVII. UNIO. — Shell transverse; hinge with an ob- 

 lique crenate tooth ; an elongated laminal tooth under the 

 ligament in one valve received into a groove in the other. 



426. U. pictorum. — Ventral margin of the shell slightly in- 

 curved ; hinge-tooth of the left valve nearly entire. 



Musculus angustius. List. Conch, t. cvxii. f. 3 — Mya pict. Linn. Syst. i. 



1112. Mont. Test. Brit. 36.— Mya ovalis and M. depressa, Dora.'Brit. 



Shells, t. Ixxxix. and ci. — Mysca ovata, Turt. Biv. Brit. 24G.— Rivers 



in England. 



Length 1 J, breadth 3 inches ; cuticle green, dusky along the layers of 



growth ; posterior extremity slightly compressed, and obliquely truncated. 



427. U. ovalis. — Ventral margin slightly rounded; hinge- 

 tooth of the left valve divided. 



Mvtilus angustior, List. An. Ang. 149. Conch, t. cxlvii. 2. — M. ovata, 



Don. Brit. Shells, t. cxxii — M. ovalis, Mont. Test. Brit. 34 Mysca 



pictorum and M. solida, Turt. Biv. Brit. 245.— llivers in England. 

 Nearly of the same dimensions as the last ; cuticle dusky or green ; more 

 tumid, and rounded anteally, than the last. 



428. M. Batava. — Shell wide, with both extremities nearly 

 equally rounded. 



]M. pictorum, Don. Brit. Shells, t. cbtxiv. — Mysca batava, Turt. Biv. 



Brit. 244 In rivers, Oxfordshire. 



Length 1 inch, breadth 2 inches ; greenish brown ; thin ; the primary tooth 

 of the left valve deeply divided. 



EXTINCT SPECIES. 



1. U. mbccmstrictus About twice as broad as long, with a constriction 



running from the front of the shell towards the beak on the posterior side, 

 the end of which is subtruncated — Soiver. Min. Conch, t. xxxiii. f. 1, 2, 3 — 

 In Ironsto7ie in the Coal formation. 



2. U. uniformis Subovate, beak near the middle of the shell ; both ends 



elliptical Sou-er. INIin. Conch, t. xxxiii. f. 4 — Middle Oolite. 



3. U. acutus Anterior side acute ; twice as wide as the other, wliich is 



blunt or rounded. Width two and a half times its lengtli — Soiver. Min. 

 Conch, t. xxxiii. f. 5, 6, 7 Middle Oolite. 



4. U, crassissimus Ovate, transversely undulated or imbricated ; beak re- 

 curved, acute ; posterior side short, round ; anterior side obscurely subcunei- 

 form; shell very thick. — Sower. Min. Conch, t. cliii — In Lias. 



5. U. Listeri Cordate, transversely imbricated, beak recurved, acute ; 



posterior side small ; middle flattish ; shell thick Sower. jNIin. Conch. 



t. cliv. f. 1, 3, 4 Inferior Oolite. 



6. U. hyhrida. — Oblong, ovate, anterior side subacuminate ; surface im- 

 bricated ; beaks recurved, acute ; shell thick. — Sower. JNIin. Conch, t. cliv. 

 f. 2. — Magnesian Limestone. 



7- U. crassiuscidus. — Oblong, elliptical, depressed ; valves thick ; surface 



