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NEW YORK STATE MUSEUM 



involution (see diagram fig. 24). Siphuncle tubular, small, extra- 

 centroventran. Surface unknown. 



Position and locality. In A 5 of the Fort Cassin beds at Yalcour. 



Observations. Only a single specimen has been found which rep- 

 resents this interesting species. From the smaller depth of the last 

 chamber we conclude that this specimen had already passed its 

 ephebic stage. With none of its congeners in the Fort Cassin beds 

 could it be confounded. It differs from the associated E . rotun- 

 d u s , to which it bears considerable superficial similarity, in pos- 

 sessing a much greater involution of the whorls and less rapid rate 

 of growth, while from E . k e 1 1 o gg i it distinguishes itself by the 

 closer arrangement of the septa (6 against 5 in the first whorl and 

 4 against 3 in the second), much greater involution, specially in the 

 third whorl and more central position of the siphuncle. 



In the great amount of involution of the last volution this species 

 resembles E . undatus from the Trenton. In the strong develop- 

 ment of the impressed zone it differs from all other congeners, ex- 

 cept the last named later form, and the early appearance of the im- 

 pressed zone (in this species already upon the first whorl) is not 

 found in any other member of the Tarphyceratidae, but is a char- 

 acter of the later Trocholitidae, which this species also approaches 

 in the reniform section of the inner portion of the ephebic whorl 

 and the smaller siphuncle. On the other hand in the Trocholitidae 

 the siphuncle is only in the younger stages ventrad of the center 

 and later on passes dorsad of the center. 



We have for these reasons retained this form among the Tarphy- 

 ceratidae, but believe that it is well advanced on the road toward 

 the Trocholitidae, the appearance of the impressed zone having by 

 accelerated development been already pushed into the nepionic 

 stage, while the siphuncle still retains its ventral position. 



The strong and increasing involution of the whorls, the absence 

 of an umbilical perforation and the early appearance of the im- 

 pressed zone, resulting from the strong involution, serve all to indi- 

 cate a strong tendency in this form toward a progressively closer 

 coiling, which contrasts with the tendency to gerontic uncoiling 

 shown by the other species of Eurystomites. The close coiling of 

 the paranepionic stage [sec fig. 5] is to such degree found only 

 in Trocholites, though in Tarphyceras it may be approached, while 

 typical Eurystomites-forms have a wide umbilical perforation. 



In the depth of the septa and the strong forward curvature of 

 the lateral sutures (sharp dorsal saddles), the sloping of the sides 



