PLATYCERAS SUBSPINOSUM HALL 177 



above the bed in which the normal representatives of this species occur. 

 In form this is identical with the smaller typical species, representing 

 merely a continuous and regular enlargement until it is more than twice 

 the size of the normal individual. The number of spines found on the 

 surface of this shell is proportional to the size, and they are scattered 

 over the whole surface. This shell therefore is merely an extremely 

 large individual of this species, developed under favorable conditions, 

 and we do not believe that it is conspecific with rarispinum of the Onon- 

 daga horizon. 



PL A TYCERAS SUBSPINOSUM HALL* 



In the coral layer at Sections C and E were found specimens which have 

 all the appearance of senescent (gerontic) individuals of P. arkonense. 

 The specimen from the brickyard, Section C, is somewhat crushed, but 

 has a length of over 40 millimeters. The young shell, up to a transverse 

 diameter of 8 or 10 millimeters, has all the characteristics of P. arkonense, 

 as found in the lowest beds at Bartlett's mills, except that the spines are 

 somewhat more numerous and crowded. The number of volutions in 

 the young corresponds closely to that found in the lower beds. The 

 transverse lines of growth are as strongly marked, or more so, than in 

 that species, and in addition to these we find fine, regular, revolving 

 lines subequidistant, and most noticeable between the strong lines of 

 growth. On certain portions of the shell the lines of growth and the re- 

 volving lines are equal in strength, thus giving a cancellated appearance 

 to the surface similar to that found on the shell of Diaphoro stoma lineala. 

 In fact, except for the spines, the first 2i volutions are identical with 

 the young Diaphorostoma lineata both in form and ornamentation. The 

 revolving striae have also been noticed on some typical specimens of 

 P. arkonense from the lower beds. 



Beyond this point the shell expands, though less rapidly than in the 

 individual before noted. The spines become few and far between ; the 

 lines of growth assume a strongly wavy appearance, indicating a sinuous 

 margin, and this becomes most strongly pronounced in a somewhat later 

 stage, where, a little to the right of the dorsum, a deep marginal notch 

 is indicated by the abruptly backward deflected lines of growth. On the 

 opposite side the lines of growth are strongly wavy, but no pronounced 

 sinuation is formed. The last 10 or 15 millimeters of the shell are with- 

 out the emargination, and the lines of growth in general are less sinu- 

 ous, becoming almost straight at the aperture. In the portion .of the 

 shell immediately succeeding the spiny stage the revolving lines are well 

 marked. These gradually become obsolete toward the front, where also 



* Compare Platyceras thetis var. subspinosum Hall. Paleontology of New York, vol. v, pt. 1, plate 3, 

 figure 30. 



