40 bulletin: museum of comparative zoology. 



Though no other cheirurid exactly dupHcates the Ceraurus pygi- 

 diuni, 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 Cheirurus qucnsiedti there are only 

 two pairs of spines, the inner pair being reduced to mere rudiments. 

 In Cheirurus 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. biniucronatus 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. hydci 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 pj^gidium, it seems that more weight should be 

 given to the cephalon then to the pygidium in determining relation- 

 ships, and Ch. hydei 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, 111. 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. 



Measuremejits: — 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 onl\' from the Niagaran near 

 Chicago, Illinois. 



