TRILOBITES. 731 
Pterygometopus callicephalus.] 
three or even four simple obscure ribs. The extremity of the axis, which is 
considerably removed from the termination of the shield, is also seen to be faintly 
bilobed. ; 
The association of these pygidia with the form of cephalon described is to some 
degree a matter of presumption, though their intimate concurrence in the same 
rocks and other accessory evidence, favors it. It gives me pleasure to dedicate this 
species to my esteemed friend, Prof. Fr. Schmidt, the founder of the genus 
Pterygometopus. 
Formation and locality—Trenton limestone, Minneapolis (Museum No. 5084); Galena horizon: Ken- 
yon (Museum No. 6768), Wykoff (Collection of Dr. Robbins); Galena shales: Cannon Falls, Minn. 
PreRYGOMETOPUS CALLICEPHALUS Hall (sp.) 
Phacops callacephalus HALL, 1841. Paleontology of New York, vol. i, p. 247, pl. LXV, figs, 3a-7. 
. The original illustrations of this species are of themselves insufficient to deter- 
mine its differential characters. An examination of the type specimens in the 
American Museum of Natural History, which have been kindly placed in my hands 
by Prof. R. P. Whitfield, has shown that in structure of cephalon and pygidium, P. 
callicephalus differs from the form which we have described at length as P. schmidiz, 
mainly in the absence of genal spines on the former. . 
Between the typical specimens referred to and separated heads and tails from 
Kenyon, Fountain and Cannon Falls, Minnesota, and Mercer county, Kentucky, the 
Fig. 51.—Cephalon of Pterygometopus callicephalus Hall. 
following points of agreement are evident. The cephalon has a short triangular 
anterior projection; genal angles broad, round and thick, without indication of 
spinules; eyes large, reaching to the occipital groove; basal glabellar lobes small and 
altogether independent; cheeks punctated beneath the eyes; surface of glabella 
Fig. 52.—Pygidium of Pterygometopus callicephalus Hall. x 2. 
coarsely tubercled. - The axis of the pygidium has incurved margins and sinuous 
annulations; of the latter there are from 8 to 10; on the pleure there are 5 duplicate 
ribs, with trace of a sixth. 
Formation and locality:x—Galena horizon: Kenyon, Fountain, Cannon Falls, Minn. According to 
Mr. Ulrich’s determination the horizon of the fossil in Mercer county, Kentucky, is Upper Trenton. 
* 
