178 SHIMKR AND GRABAU HAMILTON GROUP OF THEDFORD 



the spines have almost disappeared. This last portion of the shell com- 

 pares well with P. thetis var. subspinosum, as figured by Hall, except that 

 the strongly marked notch is not shown in that specimen. 



The specimen from the coral layer of Bartlett's mills has the original 

 form well preserved. Its earliest stages are strongly spinous, even in the 

 Diaphorostoma stage, showing thus a high degree of acceleration. The 

 spines on the surface of the later stages are more numerous than is the 

 case with the specimens from Section C. The strong emargination to 

 the right of the dorsum is well marked in the adolescent (neanic) stage 

 of this specimen, but finally becomes obliterated. The margin of the 

 adult stage is gently sinuous. Where the surface is well preserved, the 

 revolving lines of growth are visible in the adolescent stage, and there 

 are faint indications of them in the adult stage. 



A similar specimen from Eighteen-mile creek, apparently from the 

 Encrinal limestone, occurs in the collection of the geological department 

 of Columbia university. The apex of this specimen is somewhat less 

 enrolled than in those from Thedford, and the spines are fewer in num- 

 ber and most abundant on the young shell. The lines of growth have 

 a similar wavy character, and the sinuation, though less pronounced, 

 occurs to the right of the dorsum. Owing to the somewhat imperfect 

 preservation, it is impossible to say whether the revolving lines existed 

 on this specimen. Nevertheless, there are faint indications of them. 



In identifying these shells with Hall's P. thetis var. subspinosum from 

 Canandaigua lake, we propose to raise this to specific rank, since we do 

 not believe that this form has been derived from P. thetis. In fact, as 

 already noted, the derivation from P. arkonense is clear, and the direct 

 derivation of both from Diaphorostoma lineata (Conrad) is not improbable 

 in spite of the existence of spines in the " Platycerse." 



In this connection some Devonian Platycerse from Iowa are suggestive. 

 A young shell has the general form of the typical P. arkonense, except 

 that the last whorl enlarges somewhat more rapidly. The spines are 

 very few and are restricted to the dorsum. The lines of growth near 

 the aperture are wavy, there being a few faint longitudinal plications. 

 Some of the larger specimens associated with this show a series of longi- 

 tudinal plications similar to those of P. thetis, but the apex is more 

 enrolled and the aperture is more oblique. The spines have become 

 obsolete. 



With this occurs associated a perfect, regularly coiled, non-spinous 

 individual with regular lines of growth and in form a typical Diapho- 

 rostoma, except for the rapid enlargement of the body whorl. That 

 Diaphorostoma is ancestrally related to these species of Platyceras can 

 not be doubted, and since similar relations exist between Lower Devo- 



