448 



NEW YORK STATE MUSEUM 



lar ring is the bearer of four slightly curved spines, three of which are, as a 

 rule, alone seen in a compressed specimen, the fourth falling into the 

 periderm of the rhabdosome. These three easily visible spines have given 

 the species its name, which is thus seen to be a misnomer. One of these 

 spines is in fig. 419, 420 observed to extend into the sicula and therefore to 

 correspond to the virgella of the diplograptids. 



The first theca does not seem to grow 

 first in apertural direction, as in Diplograp- 

 tus and Climacograptus but directly upward 

 [see fig. 422]. 



In a few cases the nemacaulus is seen 

 to be inflated [see pi. 28, fig. 4] and some- 

 times it is broad and bandlike. 



In a subzone, observed by the writer 

 on Van Schaick island [see p. 31], which is 

 transitional from the Trenton shale to the 

 ,_. Utica shale, a mutation appears, that in its 



Fig. 419-22, cryptograptus tricornis whole character denotes the decadence and 



mm. insectiformis now r lg. 4ig, 420 biculae 



seen from opposite sides showing the apertural 1 • • • r 1 • /-p>< . 



rin ff , virgeiia and spines, rig. 421. 422 Young approaching extinction ot the species. I JUS 



rhabdt. somes. All enlarged x 5, except figure 422, 

 which is enlarged x 7 



may be described as 



Cryptograptus tricornis mut. insectiformis now 



Plate 28, figure 5 



The rhabdosome is short (8-9 mm long), rounded and broadest at the 

 sicular end (1.6 mm) and somewhat abruptly contracted to 1.2 mm, after 

 the first third of its length. The thecae are very closely arranged (16 in 

 10 mm), the sicular spines strongly curved. 



It will be seen that this mutation differs from the type of the species in 

 its shorter and more compact form, more closely arranged thecae and an 

 abrupt diminution in width from that of the typical form in the first half o\ 

 the rhabdosome. While; it bears some similarity to young specimens ot the 

 typical form, the greater closeness in the arrangement of the thecae and the 



