704 THE PALEONTOLOGY OF MINNESOTA. 



Isotelns gigas maximus. 1 



is hence an immature condition in this species. In the development of the asaphoid 

 stock, we find in the earlier Silurian Ptychopyge and Niobe this sharp lobation of the 

 caudal plate a normal character of maturity. 



Similar evidence is furnished by many genera of trilobites and may be expected 

 from all. In Homalonotus, for example, external lobation of the parts at maturity 

 regularly decreases from the appearance of the genus to its extinction. In the last 

 representative in American faunas (H. dekayi,Qi the Hamilton group) segmentation 

 of the terminal plates is almost wholly lost, but young and normally convex individuals 

 of the species are distinctly segmented, like the mature examples of H. major, from 

 the Oriskany, and H. vanuxemi, from the Lower Helderberg faunas. The genus 

 Phacops, in its restricted meaning, is conveniently divisible into species having the 

 pleural ribs of the pygidium grooved and those having them simple. The former 

 precede the latter in time. All the American Silurian and early Devonian species 

 belong to the former division, while Ph. rana of the middle and later Devonian is 

 the only representative of the latter, as well as the last member of the genus. Very 

 young individuals of Ph. rana, however, evince the duplication of the pygidial ribs. 

 Instances of this kind might be multiplied. 



Returning to the young of Isotelus maximus, we meet with a high development 

 of the genal spines, which may extend as far as the sixth thoracic segment. In 

 individuals which appear to be full grown, those in which the obsolescence of 

 segmentation is well advanced, these spines rarely pass the second or third segment. 

 This difference in size is, however, quite variable and somewhat irregularly so. In 

 Owen's species Isotelus iowensis, another form constructed on the same specific type 

 as those under consideration, the spines are represented in the restored figure given 

 by this author* as extending to the caudal shield, though the medal-ruled engravings 

 in the same work, taken from actual specimens do not indicate this length. Accept- 

 ing the restored figure as correct it appears that these long spines are associated 

 with a more distinct segmentation of the pygidium than is normal to either I. gigas 

 or the adult /. maximus; and judging from this evidence alone (I have had no 

 opportunity of examining authentic specimens of this form), this would seem to be 

 the condition of normal maturity. 



Among the Minnesota specimens is an enrolled individual conforming fully to 

 the general specific type of /. gigas-maximus in its elongate subtriangular head and 

 tail shields, and bearing a minute spinule at the genal angle, which could not have 

 extended more than half way across the first thoracic segment. This individual is 

 above the average size of the I. mnximus of the Trenton limestone. In another 



Geological Survey of Wisconsin, Iowa and Minnesota, pi. 2, fif. 3. 185?. 



