704 THE PALEONTOLOGY OF MINNESOTA. 
Isotelus gigas—maximus.] 
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, of 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 Jsotelus 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 [sotelus 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 J. 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 J. maximus of the Trenton limestone. In another 
* Geological Survey of Wisconsin, Iowa and Minnesota, pl. 2, fig. 3. 1852. 
4 
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