BEEKMANTOWN AND CHAZY FORMATIONS OF CHAM PLAIN BASIN 5OI 



cular or compressed in section. Living chamber as compared with 

 camerated part longer and larger than in most forms, less contracted 

 and with open aperture in gerontic stage. 



The genus is stated to persist from the Lower Siluric to the 

 Devonic. 



While distinctly phylogerontic, it still represents an earlier stage 

 of the family than the genera Oncoceras and Melonoceras, since 

 its aperture has not yet become contracted to any appreciable 

 amount and remains open throughout life. 



Hyatt describes the genus as exogastric ; the excellent material of 

 the genotype we have in hand shows that form to be endogastric. 

 Cyclostomiceras, as represented by its genotype, would then appa- 

 rently not lead to the Poterioceratidae but to the Phragmoceratidae. 

 The exogastric forms, now referred to Cyclostomiceras, must 

 eventually be brought under a new generic designation. 



Cyclostomiceras cassinense Whitfield (sp.) 



Plate 37, figure 1-3 ; plate 38, figure 5, 6 



Gomphoceras cassinense Whitfield. Am. Mus. Nat. Hist. Bui. 



1886. v. I, no. 8, p. 322, pi. 29, fig. 1-3 

 Cyclostomiceras cassinense Hyatt. Zittel's Textbook of Pal. 

 tr. by Eastman. 1900. 1 1530 



The collecting zeal and liberality of Prof. G. H. Perkins have 

 placed in my hands representatives of this species which are more 

 perfect than its originals. Since they allow the elucidation of im- 

 portant additional characters and since the species has gained con- 

 siderable importance by having been made the type of a new genus, 

 we avail ourselves of the opportunity of redescribing it and fur- 

 nishing drawings of the best of the specimens. 



Description. Conch a middle sized subfusiform endogastric 

 orthoceracone, which attains a size of 105 mm or more and a greatest 

 width of about 40 mm in the middle of the living chamber of geron- 

 tic specimens. Its rate of growth is large and amounts to 33 mm 

 within the space of 40 mm up to the widest part of the living cham- 

 ber, whence it decreases at a smaller rate. The section is circular 

 in the younger stages, but later on the dorsal side becomes more 

 convex than the ventral. 



The living chamber is relatively large, nearly half the length of 

 the individual, with wide open aperture in the young and mature 

 stages and slightly contracted aperture in the gerontic stage, the 

 contraction being greater on the dorsal and lateral sides than on the 

 ventral [pi. 37, fig. 3]. Apertural margin straight, transverse, 



