164 



The hinge ])lalc, which is nearly as long as the shell itself, is straight 

 above, slightly ai-cuate below, and widens outwards at each end. 

 Opposite to the beaks, or a little behind them, thei*e are a few minute, 

 crowded, granulai- denticles, arranged obscurely in three obliquely 

 ascending, but nearly transverse, rows. The whole of the teeth proper 

 are large, elongated, horizontal and nearly parallel with the cardinal 

 margin, but at their inner terminations they curve distinctly downwards* 

 In the adult shell there are three or four anterior and live posterior 

 teeth in the right valve, while in the left there are three anterior and 

 four posterior. The anterior teeth of both valves are grooved 

 transversely, but the posterior teeth are smooth. The nearer the 

 teeth are to the inner margin of the hinge plate the shorter they are, 

 and vice versa. The u])per posterior tooth is more than half the length 

 of the hinge line. 



Mr. Meek says: •'! am not well enough acquainted with the hinge of 

 this little shell to be quite siu-e that it is congeneric with the form on 

 which I proposed to found the genus Grammntodon in the " Palaeontology 

 of the Up])er Missouri." One of the casts shows a little of the impression 

 of the hinge in front of the beaks, with apj^arently four or five small 

 teeth or denticles, ranging obliquely forwai'd and upward." * 



The hinge dentition of the siib-geinis Nemodon is thus described by 

 Conrad : " Hinge line long and straight, or slightly curved under the 

 umbo; hinge in the left valve with three linear teeth parallel with the 

 antei'ior cardinal margin ; posterior lateral tooth double, very long, 

 linear ; under the apex a few granular teeth." j The number of posterior 

 lateral teeth is obviously not of generic importance, depending as it 

 does on the age or size of the shell. In Area Eufalensis, tlie type of 

 Nemodon, which is represented as about an inch in length, there appear 

 to be two posterior teeth in the left valve. In a right valve of A. Van- 

 couverensis not quite an inch and a-half long there are two long, linear 

 furrows and one short one at the posterior end of the hinge plate, 

 evideatly for the reception of two fully developed teeth, and one 

 rudimentar}' tooth in the opposite valve. There can be little doubt, 

 therefore, that the present species should be referred to Nemodon rather 

 than to Grammatodon. 



• Bulletin of the Geolog-lcal and Geographical Sui-vey of the Territories, Vol. 2, No. 4, p. 35T 

 t American Joui-nal of Conchology, Vol. v., (18T0) p. 9T. 



