416 MOLLUSCA. UNIONIDiE. 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, Don. Brit. 

 Shells, t. lxxxix. and ci. — Mysca ovata, Turt. Biv. Brit. 246. — Rivers 



in England. 



Length 1 £, 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. 



Mytilus 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 — Rivers 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. clxxiv. — 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. subconstrictus — 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. — Sower. Min. Conch, t. xxxiii. f. 1, 2, 3. 



In Ironstone in the Coal formation. 



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

 elliptical — Sower. Min. Conch, t. xxxiii. f. 4 — Middle Oolite. 



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

 blunt or rounded. Width two and a half times its length.— Sower. 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. Min. Conch. 



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



6. \J. hybrida. — Oblong, ovate, anterior side subacuminate ; surface im- 

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

 f. 2. — Magnesian Limestone. 



7* U. crassiusculus. — Oblong, elliptical, depressed ; valves thick ; surface 



