CONCHIFERA. 735 



Phalanx I. Shell regular, free, with points approximate. 



Unio BRUG. Animal (Limncea POLI) with mantle cloven, con- 

 crete 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. Branchiae 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 KAFINESQUE (Ann. des Sc. physiques de MM. BORY DE ST. VINCENT et 

 DRAPIEZ, v. pp. 287 and foil.), J. LEA (Americ. Philos. Transact., new 

 Series, in. Philadelphia, 1828, pp. 259263; in. Pt. 2, 1830, pp. 403 

 457, IV. Pt. i, pp. 63 121), and the memoir of DE FERUSSAC, in great part 

 bibliographical, in GUERIN'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 striae on the teeth 

 of the hinge, by the name of Castalia (Sp. Castalia ambigua LAM., 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 syrmatopliorm O. 

 FABR. 1 , Hyria avicularis LAM., Mya syrmatophora GRONOV. Zoophylac. 

 Tab. xvin. fig. i, Unio corrugatus BLAINV., Malacol. PI. 67, fig. i ; Hyria 

 corrugata LAM., Encycl. meih., Vers. PI. 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 HILDR. or Unio subrotundus RAFIN. of a round 

 form 2 , Unio arcceformis LEA, 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. FABRICIUS in Kongel. Danske Videnskab. SelsTcabs Naturvidenslc. Afhand- 

 linger, I. 1824, pp. 55 6r. 



3 See, for example, Unio verrucosa VALENC., in HUMBOLDT et BONPLAND Rccueil 

 d'Observ. de Zool. et d'Anat. comp. n. PI. 53, fig. i. 



