98 MOLLUSCA FROM THE CRAG. 
The difference between this genus and that of Azodonta 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 Litrora.is, Lamarck. Tab. XI, fig. 12, a—d. 
Unto tirroratts. 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 Fluy. de France, p. 133, No. 3, 
pl. 10, fig. 20, 1805. 
— — Brard. Hist. Coq. des Env. de Par., p. 222, pl. 8, fig. 6, 1815. 
— — Pfeiffer. Land and Sussw. Moll., p. 117, pl. 5, fig. 12, 1821. 
_— — Phil. En. Moll. Sic., vol. i, p. 66, and vol. ii, p. 48. 
a — 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. 
— Pranensis. Farines. 1 
— SUBTETRAGONUS. Mich. J 
— wana. Desh. 2d ed. Lam., t. vi, p. 539, No. 17, 1835. 
— antiqurior. Strickland. Silur. Syst., p. 555, 1839. 
— incurvus. Lea. Obs. Gen. Unio, vol. i, p. 107, pl. 13, fig. 27, 1532. 
— aranosus. Schum., fide Lea. 
fide Rissmasler. 
Mya ruomBorpEa. Schréter. Ausland. and Flussch., t. 2, figs. 2, 3, 1783. 
Ency. Meth., pl. 248, fig. 2. 
Spec. Char. Testa ovato-oblongd, crassd, valdé inequilaterd, antice rotundatd, postice 
subquadratd, compressiusculd ; umbonibus prominulis, flecuosis, undulatis ; dente antico 
cardinis dextri crasso, triangulart. 
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, 25 mches. Height, 1% inch. 
Locality. Cropthorn (Strickland), Clacton, Grays (Morris), Uford (Morris). 
: Recent, France, Sicily. 
Exceedingly abundant in the Fluviatile Deposit at Clacton. 
