98 MOLLUSCA FROM THE CRAG. 



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

 in the hitter, as the name impUes ; 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 Wealdcn, 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, Latuarck. Tal). XI, fig. 12, a — b. 



Unio LiTToiiAi.is. Lam. Syst. des Au. sniis Vert., p. 1 N, ISOl. 



— — Id. Hist, dcs .\n. Sans. Vert., t. vi, p. 6C, 1815. 



— — Brap. Hist. Nat. dcs Moll. Ter. et Fluv. de France, p. 133, No. .1, 



pi. 10, fig. 20, 1805. 



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



— — Pfeiffer. Land aud Sussw. Moll., p. 117, pi. a, fig- 12, 1821. 



— — Phil. Va\. Moll. Sic., vol. i, p. CC, and vol. ii, p. 48. 



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



— — Lyell, Eleni. of Geol., 2d ed., vol. i, p. fi2, fig. 29, 1841. 



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



— — Rosamas. Icon, der Land aud Sussw. Moll., No. xi, p. 1 I, t. Iv, figs. 473, 



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



— — Dvpvy. Ess. sur Ics Coq. Viv. et Foss. Dep. du Gers., p. 80. 



— PlANENSls. Farines. 1 „ , „ ■ 



(■ nde Ri'issmasler. 



— SUBTETRAGONUS. Mich. J 



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



— ANTiQi'ioR. Strickland. Silur. Syst., p. 555, 1839. 



— ixcuRVUs. Lea. Obs. Gen. Unio, vol. i, p. 107, pi. 13, fig. 27, 1832. 



— GRANosus. Schiim., fide Lea. 



Mya RllOMUOlDEA. Schriiter. .\iisland. and Fhissch., t. 2, figs. 2, 3, 1783. 

 Eucy. Meth., pi. 248, fig. 2. 



Spec. C/iar. Testa ovaio-oblongu, crassd, valde inaquilaterd, antice rotiuidaid, postice 

 mbquadrald, compressiuHCuld ; umbonibus prominulis, flexuosis, undulatin ; dente antico 

 card! ilia dextri crasso, trianf/ulari. 



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, lleiyht, I J inch. 



Locality. Crop thorn {Strickland), Clacton, Grays (Morris), Uford {Morris). 



Recent, France, Sicily. 



Exceedingly abundant in the Fluviatilc Deposit at Clacton. 



