TRILOBITES. 703 
Isotelus gigas—maximus.1 
specimens the same fact holds; the largest, and indeed the only well defined specimen 
of I. gigas that I have seen has a length of 180 mm. The smaller specimens of the 
same type, so far as their preservation permits the determination, possess cheek 
spines. 
Fig. 5.—Isotelus maximus Locke. Hudson River group, Granger. 
It would appear that among the specimens from the Hudson River group of 
Ohio, Indiana and Illinois, this is not always the case, as Locke’s immense original 
and its specific name itself indicate.* Here, locally, the occurrence of large fragments 
of spiniferous heads is not altogether infrequent, perhaps even predominant with 
reference to the aspinous heads; but whether the rule or the exception in that 
locality, it need not modify the purport of our argument. 
The morphological significance of the cheek spines in the trilobites has not 
been demonstrated, nor have they invited especial attention. Undoubtedly their 
significance varies in different groups. In this group with the evidence now at 
hand, we find a suggestion of their meaning. To present these points with the 
greater lucidity a few general remarks will be appropriate, bearing upon the 
developmental characters of the species under consideration, and upon the phylogeny 
of the asaphs. 
In Isotelus maximus, advancing growth is accompanied by gradual obscuration 
of all lobation. In the average adult of the New York Trenton, as described above, 
this obsolescence of surface division is well advanced, but in immature individuals 
the degree of lobation (especially of the parts of the pygidium, viz.: the definition 
of the axis, its annulations and the ribs of the pleur), is proportioned inversely to 
the size of the animal. Young entire individuals, 15 mm. in length, and many young 
pygidia belonging to animals not much larger have the segmentation so clearly 
developed that both ribs and annulations may be distinctly counted, the normal 
convexity of the shield being, meanwhile, undisturbed. Sharp lobation of the test 
*It might. I think, be a fair question whether the spines in this figure were not “ drawn in” from some smaller example 
retaining them. Without impugning the acumen of this observer, one cannot but be impressed with the fact that this 
original, if correctly represented, excelled in size and perfection of details all that the rocks have since afforded. 
