460 



NEW YORK STATE MUSEUM 



chamber and the preceding chamber. The specimen was obtained 

 at Pickett's Station [now called Dill], Wise. It is, in its rate of 

 growth, absolutely identical with E . k e 1 1 o g g i , nor does it ap- 

 pear to differ sufficiently in section, position of siphuncle and depth 

 of septa to warrant specific differentiation. 



Eurystomites accelerans sp. nov. 



Plate 18, fig. 2, 3 



We have a specimen from A 3 of the Fort Cassin beds at Yal- 

 cour, which in the rate of growth of the conch, the section of the 

 whorls and position of the siphuncle fully agrees — at least at a 

 certain stage, represented by the last whorl of our fragment — with 

 E . k e 1 1 o g g i ; but differs from that species by the very marked 

 flat ventral zone, the closer position of the septa (5 in 20 mm where 

 the hight of the volution is 23 mm, against 4 under the same con- 

 dition in E . k e 1 1 o g g i ) and their stronger forward curvature 

 near the line of involution. In the fiat ventral zone, the closer ar- 

 rangement of the septa and the dorsal direction of the sutures, this 

 form suggests the genus Tarphyceras as represented by T a r p h y - 

 c e r a s champ lainense, but the rate of growth of the whorl 

 prohibits a reference to that species or to the genus Tarphyceras. 

 There is little doubt that this is a new form which represents a more 

 advanced stage in the phylogenetic development of the Eurystomites 

 race than E . k e 1 1 o g g i . This is shown specially in the early ap- 

 pearance of the ventral zone and the greater amount of curvature 

 of the suture which here in the neanic stage is already much greater 

 than that in the ephebic stage of E . k e 1 1 o g g i . A certain amount 

 of acceleration in the development of the characters as compared 

 with the latter species has therefore taken place. 



E . Virginian us also has more numerous sutures than E . 

 k e 1 1 o g g i , but these are described as straighter than those of 

 E. kelloggi in all stages. In this particular character it is 



cameras, which is greater in E. accelerans (at end of 

 2 volutions, 4 cameras in space of 10 mm against 5 in E. vir- 



Fijr. 23 Eurysto- 

 mites acceler- 

 ans sp. nov. Trans- 

 verse section. x," 



hence the direct opposite of E. accelerans. 

 A direct comparison of the type of the latter 

 species with those of E. virginianus, 

 kindly forwarded by the United States National 

 Museum, has shown that the two forms differ 

 in their rates of growth, E. virginianus 

 having the greater rate; in the depth of the 



