BEEKMANTOWN AND CHAZY FORMATIONS OF CHAMTLAIN BASIN 469 



The differences in the development of this species and the very 

 similar T . s e e 1 y i have been pointed out by us under the latter 

 species. The adults differ, according to the original descriptions of 

 this species and that of T . s e e 1 y i , " in having a smaller number 

 of volutions in the finished shell, and in their being very slightly 

 more expanding and less compressed laterally. But the principal 

 distinction and the only one which can be relied upon for the sepa- 

 ration, is the expanded aperture." In the absence of this feature, 

 it is added by the same author, that " it is barely possible to distin- 

 guish ' the species ' ". As we have noted before, the development of 

 the two forms is different. The earlier appearance and stronger 

 development of the ventral flattening, as well as the earlier and more 

 rapid turning of the siphuncle toward the center in T . s e e 1 y i 

 appear to suggest that this is the more advanced form and T . 

 champlainense the more primitive of the two. Also the 

 stronger development of lateral zones in the ephebic whorl of T . 

 s e e 1 y i would point to the same inference. 



Tarphyceras perkinsi Whitfield (sp.) 



Nautilus perkinsi Whitfield. Am. Mils. Nat. Hist. Bui. 1897. 

 9:182, pi. 5, fig. 1, 2 



This species, which has been described a considerable time after 

 the other cephalopods of the Fort Cassin form, is based upon two 

 specimens. I have not observed any more representatives of the 

 same in the collections which have come under my notice and have 

 therefore nothing to add to the original description. 



It is stated in the latter that " this species is somewhat closely 

 related to Nautilus champlainensis from the same beds, 

 but differs principally in the presence of the oblique undulations of 

 the surface." Since lower undulations are also found on the mature 

 living chamber of T . champlainense and both forms agree 

 completely in rate of growth, amount of involution and depth of the 

 chambers, the presence of strong ribs upon the type specimen of T . 

 perkinsi is evidently to be looked upon as constituting merely a 

 further development of the corresponding character of T . c h a m - 

 p 1 a i n e n s e ; and the difference is one of grade only and would not 

 appear to be of more than varietal value. The sutures in the 

 types of T . perkinsi are straight transverse and in that of T . 

 champlainense arching forward upon the dorsal side ; but still 

 this difference is not absolute since in the former species the 

 sutures in the earlier volutions also possess a stronger dorsal saddle. 



