292 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



The apex of the only specimen jet obtained being a little broken, I have 

 not been aide to determine whether or not it was abruptly attenuated and 

 curved backward, though it appears to have been, and doubtless was, so 

 curved. No internal casts of this species have yet been seen, consequently 

 nothing is known of the nature of its muscular impression; but, from the 

 appearance of the shell in other respects, it is believed that it will be found 

 to possess the internal characters of this genus. 



The side-view of this form was inadvertently numbered 5, d, on 

 plate 18. 



Locality and position. — Same as last. 



A bi ■ s o in y o ii alveolus, M. & H. 



Plate IS, figs. 4, a, b. 



Helcion alveolus, Meek and Haydei! (185G), Proceed. Acad. Nat. Sei., VIII, 08. 



Anisomyon alveolus, Meek aud Hayden (1860), Am. Jour. Sci. aud Arts, XXVIII (2d ser.). 35. 



Shell small, subovate-patelliform, a little less than twice as long as wide, 

 very thin, slightly broader behind than toward the front; extremities rather 

 narrowly rounded; lateral margins convex in outline; summit depressed, 

 nearly central or very slightly behind the middle; lateral slopes from the 

 apex a little convex, and diverging at an angle of about 80°; posterior and 

 anterior slopes nearly equally convex, and converging toward the summit at 

 an angle of about 120°; surface smooth er only marked by obscure lines of 

 growth. 



Length, 0.63 inch : breadth, 0.42 inch 



Internal casts of this species show that the muscular impression agrees 

 in all its essential characters with that of A. pateUiformis ; though the anterior 

 extremities are narrower in proportion to the size of the shell than in that 

 species, and the slender part on the right posterior side seems to be always 

 continuous. 



It is possible that this may be the young of the last, though it is more 

 narrowly rounded at the anterior extremity than that species seems to have 

 been at any stage of its growth, while its shell is much thinner, and the apex 

 more nearly central. I am, therefore, with the present means of comparison, 

 compelled to keep them separate. 



Locality and position. — Same as last. 



