124 NEW YORK STATE MUSEUM 



The described American species most like this new form is 

 Ceraurus dentatus Raymond and Barton (Mus. Comp. 

 Zoology Bui., v. 54, no. 20, p. 534, pi. 1, fig. 2, pi. 2, figs. 4, 5. 

 1913). In this species both the second and third pairs of spines 

 on the pygidium are represented by denticles which cross the pos- 

 terior border, the chief difference between the pygidia of the two 

 species being thus the suppression of the second pair of pygidial 

 spines in C. dentatus. The glabella of C. dentatus is 

 similar in shape to that of the present species, but is somewhat less 

 depressed and has coarser tubercles. 



The only other species of Ceraurus in the Chazy are Ceraurus 

 hudsoni Raymond (Annals Carnegie Mus. v. 3, no. 2, p. 367, 

 pi. 14, fig. 15. 1905), a species known only from specimens of the 

 cephalon, and C. granulosus Raymond and Barton, a species 

 with a small cephalon and with granulose rather than pustulose 

 surface (Mus. Comp. Zoology Bui., v. 54, no. 20, p. 536. 1913). 

 The cephalon of C. hudsoni is much like that of Ceraurus 

 pleurexanthemus, with which species it might have been 

 united at the time of the original description had it not been for the 

 possibility that the pygidium might prove to be of a very different 

 type. 



Ceraurus ruedemanni would appear to be a derivative 

 of the Russian Ceraurus scutiger by the suppression of 

 the third pair of spines on the pygidium, a shortening of the central 

 spine, and the reduction in size of the pustules of the surface 

 ornamentation. The Kuckers formation, which contains the Rus- 

 sian species, is the second of the formations of C, the formations 

 characterized by Echinosphaerites. If we are to judge by com- 

 parison of the sections in Russia, Norway, and the eastern United 

 States, the Kuckers is not widely different in age from the Chazy 

 of the Champlain valley. 



The species here described is also of especial interest as an 

 example of another link in the chain connecting Ceraurus 

 pleurexanthemus and later species of Ceraurus with the 

 theoretical Ceraurus ancestor. The pygidium of Ceraurus 

 pleurexanthemus is small, with a much reduced central 

 portion, and two very long, strong spines at the sides. The pos- 

 terior margin of the portion between the great spines is often 

 smooth, but as Raymond and Barton have shown, it may be 

 undulating or even show small denticles. Although such a pygi- 

 dium appears to be highly specialized, the variations in the posterior 

 margin might suggest that it was really a variable form which 



