286 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



suspected. At one time I was rather inclined to think that there might have 

 been a minute perforation at the immediate point of the summit, owing to the 

 fact that one of the specimens of A. borealis has a little opening there, with 

 apparently smooth margins; while some of the others show some indications 

 of such a perforation. The fact, howeyer, that in all of the latter there is 

 unmistakable evidence of fracture of the point, while in one specimen of 

 another species (see fig. 5, a, pi. 18) this little retlexed apex of the summit 

 comes to an abrupt point, without the slightest opening, shows, as is now 

 believed, that this is always the case, where the point is not accidentally 

 broken or worn off. So very minute a natural opening at that point would 

 also be against all analog} in such a shell; while the delicacy and the pecu- 

 liar hooked form of this little apex would render it especially liable to acci- 

 dental fracture. 



In two of the species, there are six equidistant impressed hair-lines 

 radiating from the apex down the lateral anterior and posterior slopes, nearly 

 or quite to the margins : and in another species, only known from its internal 

 cast, six similarly-disposed, rather deep, and wider furrows extend, from the 

 apex to the margins, to which they impart slightly emarginate appearances at 

 their terminations (see fig. 8, a, b). As nothing of this kind, however, is seen 

 on some of the other species, apparently agreeing in all other respects except- 

 ing mere specific details, I cannot believe this a generic character. 



From the foregoing description and the accompanying figures, it will be 

 seen that this group consists of shells more or less nearly resembling in form 

 the well-known genus Patella. But on closer examination, the conchologist 

 will readily see that they present marked and important differences. In the 

 first place, they are all thinner and smoother shells than we generally see in 

 that genus ; but the most important distinctions consist in the nature of the 

 muscular scar and the abruptly pointed and retlexed apex, the former of 

 which at least, indicates fundamental differences in the organization of the 

 animal. 



The peculiar interruption in the muscular impression on the right side, 

 together with the limpet-like form of these shells, also suggests more or less 

 near relations to the genus Siphonaria. They differ, however, at least gen- 

 erically, from that group, in having the interruption in the right posterior 

 lateral, instead of the right anterior lateral region, and in the pointed and 

 abruptly retlexed apex. They are also smoother and thinner shells than most 



