CONCHIFERA. 735 
Phalanx I. Shell regular, free, with points approximate. 
Unio Bruce. Animal (LZimnea Pout) with mantle cloven, con- 
erete posteriorly by a commissure about the anus, surrounding the 
anus with a trachea like an aperture. Posterior margin of each 
lobe of mantle furnished with cirri or tentacles. Branchie of each 
side coalesced behind the foot. Shell thick, pearly within, with 
points most frequently decorticate. Hinge mostly with two teeth 
in each valve, the anterior thick, obliquely striated, the posterior 
compressed like a lamella, elongate, sometimes obsolete. (Teeth 
of left valve bipartite by a groove for receiving the teeth of the op- 
posite valve.) 
This genus is very numerous in species, especially in North America, 
The knowledge of these species is attended with great difficulties, since the 
naturalists-who have described them have dispersed their descriptions, 
partly in Magazines, which are entirely unknown in Europe, and at the 
same time have paid little regard to the contemporary or previous labours 
of one another. We mention here only, as the most accessible, the papers 
~ of RAFINESQUE (Ann. des Sc. physiques de MM. Bory DE St. VINCENT et 
DrRarizzZ, v. pp. 287 and foll.), J. Lea (Americ. Philos. Transact., new 
Series, 111. Philadelphia, 1828, pp. 259—263; ul. Pt. 2, 1830, pp. 403— 
457, Iv. Pt. 1, pp.63—121), and the memoir of DE Férussac, in great part 
bibliographical, in GuéRIN’s Magasin de Zoologie, 1835, in which the 
descriptions of TH. Say and others are cited. 
The form of the numerous shells of this class is very different. For some 
of the forms distinct genera have been adopted. Thus Lamarck distin- 
guishes the species with a triangular shell and transverse strize on the teeth 
of the hinge, by the name of Castalia (Sp. Castalia ambigua LamM., BLAINV., 
Malacol. Pl. 67, fig. 4, from S. America). The species that have an oblique 
shell, and at the posterior margin present a large, flat, triangular pro- 
longation, he unites under the genus Hyria. (Sp. Unio syrmatophorus O. 
Fasr.t, Hyria avicularis Lam., Mya syrmatophora Gronov. Zoophylac. 
Tab, xvi. fig. 1, Unio corrugatus Buatnv., Malacol. Pl. 67, fig. 1 ;—Hyria 
corrugata Lam., Encycl. méth., Vers. Pl. 247, fig. 2.) The other species 
LAMARCK leaves together under the genus Unio. But besides Castalia and 
Hyria great varieties of form are found, of which sub-genera might be 
made, as Unio orbiculatus Hitpr. or Unio subrotundus Ra¥in. of a round 
form?, Unio arcaformis LE, with a flat broad surface behind the points, 
&c. In European species the shell is usually less different in form, elongate, 
forwards round and obtuse, backwards pointed and prolonged. 
1 0. Fasricius in Kongel. Danske Videnskab. Selskabs Naturvidensk. Afhand- 
linger, I. 1824, pp. 55—61. 
2 See, for example, Unio verrucosa VALENC., in HumBoLpDtT et BONPLAND Recueil 
WObserv. de Zool, et d’Anat, comp. 11. Pl. 53, fig. 2. 
