TRILOBITES, 705 
Tsotelus gigas—maximus,] 
specimen from the Trenton horizon of New York of about the same size as the 
foregoing there is no spine, but at the angle of the cheek there is a distinct puckering 
Fig.6. Genal extremity of Zsotelus gigas maximus, with minute spinule, x 3. 
of the test, making a small, acute tubercle. This evidence goes to indicate that the 
cheek spine in these asaphids is a character of immaturity, diminishing in size as 
the adult condition is approached. 
Fig. 7.—Genal extremity of Isotelus gigas-maximus, with puckered, but aspinous apex. Trenton 
limestone, N. Y. e 
In the earlier representatives of this generic type where segmentation of the 
pygidium is retained at maturity (Ptychopyge), the long cheek spines are also retained, 
e.g. Asaphus romingeri Walcott, of the Black River limestone; and, as observed above, 
the adult condition of such species is to be regarded as phylogenetically immature. 
The sporadic or restricted local appearance of genal spines in large individuals, an 
occurrence of rarity, is a natural exception to normal processes, the retention of 
infantile characters at maturity, or their resumption in the senile condition, occurring 
alike in individuals, species and races. The Jsotelus canalis Whitfield, sp., is an early 
Trenton (Birdseye) or Calciferous form, very closely similar in all specific values to 
T. gigas-maximus, retaining, however, at maturity cheek spines, without the segment- 
ation of the cephalon and pygidium. Our acquaintance with this form is essentially 
restricted to the single large, enrolled, distorted and otherwise imperfect specimen 
described’ by Mr. R. P. Whitfield. Toward the close of the Trenton epoch and during 
the predominance of the normal Jsotelus-type and with the senile decline of the race, the 
highly segmented and spiniferous type reappears in the Asaphus canadensis Chapman 
and the Gerasaphes ulrichana gen. et. sp. nov., both from the horizon of the Utica slate, 
both recurrent Ptychopyge or a senile reappearance of the immature individual type.* 
*Some years ago the writer had the opportunity of studying the structure and mode of development of the eye ina 
single highly facetted trilobite, Phacops rana (Journal of Morphology, vol. ii, p. 253, pl, 21. 1883), and demonstrated not only 
a gradual increase in the number of lenses from immaturity to adu!thood, but also that, after maturity, senile growth was 
accompanied by a resorption or obscuration of the lenses, and in consequence, by a return of the visual area to its infantile 
condition. These conclusions were derived from the investigation of a vast number of selected specirmens, 
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