308 GEOLOGICAL SURVEY OF THE TERRITORIES. 



Fr. Ill, PL 1-4), from the Neocoinian ; but it is rather more coarsely stri- 

 ated, and wants the posterior dorsal impression of that shell, from which 

 it also differs in being less evenly convex. 

 Locality and position : Same as last. Discovered by Professor Mudge. 



MACTRA? CANONENSIS, (MEEK.) 



Shell small, very thin, transversely subovate, rather compressed or 

 moderately convex, with length about once and a half the height; 

 anterior side rounded; posterior side longer, narrower, and obliquely 

 subtruncated at the extremity; pallial margin forming nearly a semi- 

 ovate curve, being most prominent anteriorly, straight or very slightly 

 sinuous behind the middle, and rounding up very abruptly to the lower 

 part of the truncated posterior margin; dorsal outline nearly straight 

 and sloping to the truncated posterior, and declining more abruptly in 

 front; beaks small, moderately prominent, and placed one-third the 

 length of the valves from the anterior margin; posterior umbonal 

 slopes rather prominent to the posterior basal extremity, while the sides 

 in front of this prominence are flattened, or even very slightly concave 

 below. Surface with rather regular but distinct lines and farrows ot 

 growth. Muscular impressions shallow ; posterior round-oval; anterior 

 narrower, with a slender prolongation above; pallial line with a shallow, 

 rather rounded sinus. 



Length, 0.78 inch; height, 0.53 inch; convexity, 0.31 inch. 



At the same time that I refer this species, for the present, to the 

 genus Mactra, I really do not believe that it belongs properly to that 

 genus, as restricted by late authorities on conchology. I merely place 

 it there, however, because its hinge is unknown, and it presents some 

 characters indicating relations to that group. In form it is very unlike 

 Mactra, and more nearly resembles Grassatella. Its thinness and sinu- 

 ous pallial line, however, at once forbid its reference to that group, 

 while the latter character, and especially a triangular fissure seen in the 

 hinge margin between the beaks, as in Mactra (IScIiizodesma), indicate 

 relations to that genus. Its hinge, however, will probably be found 

 presenting characters that warrant its separation as a distinct genus, 

 judging from its general physiognomy. 



Locality and position : Canon City. 



genus ARCOPAGELLA, (meek.) 



Shell equivalve, more or less nearly equilateral, longer than high, 

 with margins closed all around and -smooth within. Hinge with two 

 cardinal, .and one anterior and one posterior lateral teeth in each valve, 

 left valve with anterior cardinal tooth larger than the posterior and 

 trigonal in form, but sometimes rather deeply emarginated below, 

 placed directly under the beak; posterior cardinal tooth small, slender, 

 and ranging obliquely backward and downward close to the larger one, 

 so as to leave only a slender pit between, corresponding to another on 

 the anterior side of the principal cardinal tooth, which two pits are for 

 the reception of the cardinal teeth of the right valve; anterior and 

 posterior lateral teeth both elongated parallel to the cardinal margin, 

 the former approaching more nearly to the cardinal teeth. Right valve 

 with, under the apex, two diverging, slender cardinal teeth, like the 

 posterior one of the other valve, with a triangular pit between them for 

 the reception of the principal cardinal tooth of the other valve ; ante- 

 rior one more oblique than the other, and nearly or quite connecting 



