S82 TESTACEOUS MOLLUSCA. 



or less arcuated ; umbones eroded : nacre purple ; cardinal teeth 

 solid, trigonal, jagged ; lateral straight. l T 7 ^...2i. Mexico. The 

 specimen delineated appears an abnormal form, being produced and 

 bluntly subcuneiform in front, and very short and convex behind : 

 judging from the lines of growth this is not always the case. The 

 V. Crocodilorum of Morelet is nearly allied, but the shape is 

 oblong, the surface grooved behind, and the teeth less solid and 

 triangular. 



Unio collinus (t. 20, f. 57), Conr. Mm. U. t. 36, /. 2. Suboval, 

 slightly subtrapeziform when mature, subinequilateral, not solid, at 

 most subventricose, not distinctly rayed ; when young ochraceous, 

 smooth, and armed in the umbonal region with a few short spines 

 behind the inconspicuous umbonal ridge ; when aged olivaceous- 

 brown, unarmed, with concentric wrinkles of increase ; ventral edge 

 changing from convex to retuse, subparallel with the slightly convex 

 front dorsal margin ; front extremity broadish, angulated dorsally, 

 rounded ventrally ; hinder extremity unsymmetrically rounded ; 

 umbones eroded (in the adult), not prominent ; nacre tinged with 

 flesh-colour; cardinal teeth oblique, 2 in each valve. 1£...2. 

 Virginia. 



Unio scobina (t. 23, f. 40), Benson, MS. Oblong, inequilateral, 

 moderately thick, rather compressed, with a dark green ray on the 

 anterior slope, densely covered with raised corrugations, which, upon 

 the simple umbonal slope are disposed in somewhat transverse diver- 

 gent curved narrow folds, are angularly met beyond it by similar 

 raised wrinkles, are arranged upon the hinder extremity in short sub- 

 transverse angularly flexuous lines, and elsewhere are broken into 

 short zigzags, or angular dots (like a worn out rasp) ; dorsal and ven- 

 tral edges subparallel, the latter straightish, a little retuse ; anterior 

 side bluntly biangulated ; umbones eroded, not prominent ; nacre 

 bluish, iridescent in front ; anterior lateral tooth elongated, posterior 

 lateral tooth very short, much elevated. T 7 o ... If, Assam. The 

 primary tooth in the only valve known to me is a mere callus. 



Unio radula (t. 23, f. 41), Benson, MS. Transversely oboval, 

 inequilateral, not very thick, rather compressed, olivaceous, sculptured 

 as in U. scobina, but rather more linearly and transversely ; ventral 

 and front dorsal edges convex and subparallel ; hinder extremity 

 rounded, narrower : umbonal ridge inconspicuous, umbonal slope 

 simple ; neither the eroded umbones nor the ligament prominent ; 

 nacre bluish ; umbonal cavity shallow ; hinder tooth strong, compli- 

 cated ; front lateral tooth elongated, a little curved. } ... 1-J-. 

 Assam. 



Unio cucumoides (t. 24, f. 4), Lea, Trans. Am. Phil. Soc. 8, t. 7, 

 /. 2; on U. 3, p. 80. Very elongated, subcylindraceous, scarcely 



