98 MOLLUSCA FROM THE CRAG. 



• 



The difference between this genus and that of Anodonta is in the absence of teeth 

 in the latter, as the name implies ; but the modification of the hinge as well as the 

 outward form of the shell in the whole group is so exceedingly variable, some being 

 perfectly orbicular, while others are elongated and cylindrical, as to have caused it to 

 be divided into numerous Genera, but these divisions appear to be founded upon 

 characters alike unstable and insignificant. 



As fossil, this genus is said to date its existence from the Coal Measures ; it is 

 found in the Wealden, and in the Fresh-water Deposits of the Older as well as the 

 Newer Tertiaries, and wherever met with, the species are seldom numerous, but the 

 individuals are usually very abundant. 



1. Unio littoralis, Lamarck. Tab. XI, fig. 12, a — b. 



Unio littoralis. Lam. Syst. des An. sans Vert., p. 114, 1801. 



— — Id. Hist, des An. Sans. Vert., t. vi, p. 66, 1815. 



— — Drap. Hist. Nat. des Moll. Ter. et Fluv. de France, p. 133, No. 3, 



pi. 10, fig. 20, 1805. 



— Brard. Hist. Coq. des Env. de Par., p. 222, pi. 8, fig. 6, 1815. 



— — Pfeifer. Land and Sussw. Moll., p. 117, pi. 5, fig. 12, 1821. 



— — Phil. En. Moll. Sic, vol. i, p. 66, and vol. ii, p. 48. 



— — Mag. Nat. Hist., New Ser., vol. ii, p. 548, fig. 27, 1838. 



— — Lyell, Elem. of Geol., 2d ed., vol. i, p. 62, fig. 29, 1841. 



— — G. B. Sow., Jr. Conch. Man., fig. 145, 1842. 



— — Rossmas. Icon, der Land and Sussw. Moll., No. xi, p. 14, t. lv, figs. 473, 



744, and No. xii, p. 27, t. lvi, figs. 752—754, 1844. 



— — Dupuy. Ess. sur les Coq. Viv. et Foss. Dep. du Gers., p. 86. 



— Pianensis. Farines. 1 , 



( fide libssmasler. 



SUBTETRAGONUS. Mich. J 



— nana. Desk. 2d ed. Lam., t. vi, p. 539, No. 17, 1835. 



— antiquior. Strickland. Silur. Syst., p. 555, 1839. 



— incurvcs. Lea. Obs. Gen. Unio, vol. i, p. 107, pL 13, fig. 27, 1832. 



— granosus. Schum., fide Lea. 



Mya rhomboidea. Schroter. Ausland. and Flussch., t. 2, figs. 2, 3, 1783. 

 Ency. Meth., pi. 248, fig. 2. 



Spec. Char. Testa ovato-oblongd, crassd, valde incequilaterd, antice rotundatd, postice 

 subquadratd, compressiusculd ; umbonibus promimdis, Jlexuosis, undulatis ; dente antico 

 cardinis dextri crasso, triangulari. 



Shell ovato-oblong, thick, very inequilateral ; anterior side rounded, posterior 

 subquadrate, slightly compressed ; umbones with undulating rugosities ; anterior 

 cardinal tooth in the right valve thick and triangular. 



Length, 2\ inches. Height, \\ inch. 



Locality. Cropthorn {Strickland), Clacton, Grays {Morris), Ilford {Morris). 



Recent, France, Sicily. 



Exceedingly abundant in the Fluviatile Deposit at Clacton. 



