176 SHIMER AND GRABAU HAMILTON GROUP OF THEDFORD 



depressed or flattened fold are also retained for a long time. These in- 

 dividuals of the coral layer therefore represent the connecting forms 

 between the typical S. mucronatus and the shortened variety thedfordensis. 



PLATYCERAS ARKONENSE VAR. NOV.* 



The above name is proposed for the small spiny species of Platyceras 

 which characterizes the lower beds of the Hamilton group at Bartlett's 

 mills and which has generally been considered identical with Hall's 

 P. dumosum rarispinum of the Onondaga beds. 



This species is characterized by its closely incurved apex, which has 

 the features of the young Diaphorostoma, and may be referred to as the 

 Diaphorostoma stage. It comprises about two volutions, the last of 

 which, however, expands much more rapidly than the corresponding 

 one in Diaphorostoma. The lines of growth in the early stage are per- 

 fectly regular, and in most cases spines do not appear until the Diapho- 

 rostoma stage is past. A small specimen, having the form and degree 

 of coiling of this species, but without spines, was found in the lowest 

 bed at Bartlett's mills. What may be considered the adult portion of the 

 whorl is broadly expanded, the aperture varies from oval to irregularly 

 rounded, with a diameter sometimes exceeding 20 millimeters. The 

 surface of the adult portion of the shell is furnished with tubular spines, 

 which in some specimens have been found to have a length of 8.5 milli- 

 meters, not counting the apex, which was broken off, and would have 

 given a total length of perhaps 10 millimeters. The lines of growth on 

 the adult portion of the shell are somewhat sinuous, though no pro- 

 nounced emargination has been observed in any of the specimens. 



The characteristic features of this species are the closely and regular^ 

 incoiled beak (Diaphorostoma stage), and regular^, though rapidly, 

 expanding adult portion of the shell with its strongly marked spines. 

 The differences between this and the typical rarispinum from the Onon- 

 daga limestone will be apparent on the inspection of the figures given 

 by Hall.f The largest adult specimens of this species observed are about 

 25 millimeters in length, though old-age individuals of much larger size 

 occur, as noted beyond. From the abundance of this species and its 

 uniform size and characters, we must assume that these specimens rep- 

 resent the normal adult characteristics, and that they are not young in- 

 dividuals of a type which normally is of a larger size. 



A large individual, something over 40 millimeters in length, and the 

 aperture, which is subquadrate, about 30 millimeters in diameter, has 

 been found in the coral layer of the same locality — that is, 28 to 30 feet 



* 1874, Platyceras dumosum var. rarispinosum Hall (?) Nicholson; Paleontology of Ontario, page 

 117, figure 52. 

 t Paleontology of New York, vol. v, pt. 2. 



