47© NEW YORK STATE MUSEUM 



which soon disappears, and in the other species the dorsal saddle 

 is likewise little developed in the last sutures. It is, hence, evi- 

 dent that also this apparent difference is due to a little earlier 

 development of a feature in one of the forms. 



In view of the slight differences between T . p e r k i n s i and T . 

 champlainense, which consist only in the somewhat stronger 

 and earlier development of features present in both, a specific sepa- 

 ration of the two would seem unwarranted, were it not for the fact 

 that these differences apparently happen to lie along lines of phy- 

 logenetic development to a new group, or express a tendency to a 

 development of a new group and thereby gain greater phylogenetic 

 importance and invite recognition. 



The tendency to greater prominence of the costae and the tachy- 

 genetic pushing back of the time of their appearance on the whorls 

 distinctly points to the Plectoceratidae. The prevalence of the strong 

 ribs in T. p e r k i n s i might even suggest the assignment of this 

 variety to the genus Plectoceras. Since, however^, the Plectocera- 

 tidae are diagnosed by Hyatt as forms in which the annular costae 

 appear in the neanic stage and here they are not observed earlier 

 than in the ephebic stage, this form still falls within the confines of 

 Tarphyceras. 



Tarphyceras clarkei sp. nov. 



Plate 22 



Description, Conch a loosely coiled nautilicone which in the 

 gerontic age becomes uncoiled, the evolute portion separating but 

 little (about its own hight at the aperture) from the rest of the conch. 

 4-5 volutions are found in the adult form giving the conch a diam- 

 eter of 170 mm or more. Volutions in early stages subcircular, in 

 ephebic and gerontic stages compressed elliptical with somewhat nar- 

 rower abdomen [see text fig. 28-31], ratio of hight to width at end 

 of third volution approximately as 6:5, in second whorl as 8:7. 

 Ventral zone indicated by lower convexity in later whorls, impressed 

 zone by a slight flattening, which persists on greater portion of free 

 whorl (or throughout ?). Rate of growth of conch slow (that of 

 hight about one third within one whorl). Umbilical perforation not 

 observed. 



Living chamber occupying one half of one volution in type speci- 

 men, but probably longer, free in gerontic age. Aperture not known, 

 by indications from growth lines much advanced in dorsal region, 

 and uniformly receding in ventral direction. Cameras shallow, at- 



