40 BULLETIN: MUSEUM OF. COMPARATIVE ZOOLOGY. 
Though no other cheirurid exactly duplicates the Ceraurus pygi- 
dium, there are numerous cases among the trilobites referred to the 
genus Cheirurus in which there is a reduction of the inner pairs of 
spines of the pygidium. Thus, Weller has figured a Cheirurus pygi- 
dium from Lemont, near Chicago, in which spines of the inner pair 
are shorter than the others. In Chetrurus quenstedti there are only 
two pairs of spines, the inner pair being reduced to mere rudiments. 
In Chetrurus hawlei there is a still further reduction, so that there is 
only one pair of long spines, thus producing a pygidium which is a 
parallel to that of Ceraurus, though differing considerably from it in 
detail. ‘The typical number of segments in a Cheirurus pygidium 
seems to be five, a protopygidium and four pairs of coalesced segments 
which originally had free spines. Among the species referred to 
Cheirurus by Barrande may be seen Ch. minutus Barrande with four 
pairs of spines, Ch. bifurcatus with four pairs, the central pair partly 
united, Ch. insignis and many others with three pairs and a central 
spine. In England, Ch. bimucronatus with three pairs without the 
central spine. In Bohemia again, Ch. quenstedti Barrande with two 
pairs of spines and two rudiments, Ch. hawlei Barrande with one pair 
spines and four rudiments, and in America Ch. hydet Weller and Ch. 
nuperus Billings with one pair of spines and three rudiments. Differ- 
ences in the cephalon show that this is not a progressive (or regressive) 
series, but apparently a number of cases of parallel development by 
the loss of the posterior inner pairs of spines. 
In view of this general tendency among the cheirurids to a reduction 
of the spines of the pygidium, it seems that more weight should be 
given to the cephalon then to the pygidium in determining relation- 
ships, and Ch. hydet and Ch. nuperus are therefore referred to Cheiru- 
rus. It may be proper, when the family has been more fully studied, 
to erect a new genus for these peculiar species. The M. C. Z. has 
recently acquired a fairly complete specimen (M. C. Z. 631) of this 
species, of which only two other specimens are now known. The 
specimen is from an unknown locality near Chicago, Ill. This speci- 
men shows that the eye is very far forward, opposite the first pair of 
glabellar furrows. Both the genal and pygidial spines are longer than 
had been supposed, and as pointed out above, the pleura of the thorax, 
beyond the line of nodes denoting the fulcra, are free blade-like spines. 
Measurements: —'The specimen figured by Weller is 24 mm. long, 
14.5 mm. wide at the genal angles, and the glabella is 5 mm. wide at 
the back. 
Formation and locality: — Known only from the Niagaran near 
Chicago, Illinois. 
