BEEKMANTOWN AND CHAZY FORMATIONS OF CHAMPLAIN BASIN 455 



and ventral saddles and the lateral lobes as in other species of this 

 genus. 



The volutions are in contact, but no contact furrow was formed 

 at any age. The contact takes place as in the young of Estonio- 

 ceras perforatum [fig. 9, pi. 7] on the venter of the para- 

 nepionic volution. 



The volution in the neanic stage, dorsoventral diameter 13 mm, 

 has a much narrower venter in proportion to the dorsum than in the 

 adult. The venter was rounded at all stages and also the dorsum. 

 The ananeanic and nepionic stage were not present in the original 

 specimen in the Museum at Ottawa, but in following out the same 

 lines it is easily ascertained that the umbilical perforation must have 

 been enormous, at least 15-17 mm in diameter. The living chamber 

 was somewhat over one fourth of a volution in length. The whole 

 diameter was about 108 mm. It was reported as having been fovmd 

 in the Chazy limestone. 



A large gerontic specimen which we have collected on the east 

 shore of Valcour island, in the dove-colored Upper Chazy lime- 

 stone, furnishes the following additional data. 



The conch, which on the whole seems to be well characterized by 

 Billings in being described as " discoid planorbiform ", is composed 

 of not more than four volutions, if the umbilicus has the large size 

 inferred by Hyatt, and it attains a diameter of 165 mm or more. 

 The whorls are very slender and gradually expanding, the rate of 

 growth being i :2.2 in one volution ; the dorsoventral diameter in- 

 creasing from 10 mm to 22 mm. The siphuncle is also very slender 

 or narrow (1.4 mm in third volution). The living chamber ap- 

 parently reaches one half of a volution or more, and becomes 

 slightly evolute in the gerontic growth stage. The cameras are 

 shallow, their depth being 4 mm at the end of the second volution 

 and 5.5 mm at the end of the third volution; the septa are very 

 concave, their depth equalling that of i^ cameras. 



The surface, which is not shown in the type, is exposed on the 

 third and fourth volutions of our specimen. The third volution 

 shows strong ribs or costae like those of Plectoceras ; they curve 

 forward on the sides and disappear toward the end of the third 

 volution, so that the last volution possesses fine growth lines only. 

 From their appearance at the beginning of the third volution, 

 there is no doubt that the second volution bore also ribs on its 

 greater part. 



From Plectoceras jason, another costated nautiloid of the 

 Chazy rocks, this species can be readily distinguished by the more 

 slender volutions, the gyroceran form of involution, the volutions 

 being only in contact and the earlier cessation of costation ; in sec- 



