532 bulletin: museum of comparative zoology. 



The only important variations noted have been in the character of 

 the portion of the pygidium inside the great spines. The posterior 

 border of this part of the pygidium may be either aspinose and rounded, 

 or have 2, 4, or 5 short spines or denticles. 



Figs. 1-3 illustrate the three types of pygidium found among the specimens of 

 Ceraurus pleurexanthemus at Trenton Falls, N. Y. Some, as in fig. 1, show 

 no traces of spines between the great spines at the sides; others, fig. 2, have 

 one pair of spines; while those like fig. 3 have a suggestion of two pairs of 

 spines between the great spines. A fourth type of pygidium, which is not 

 illustrated, is represented by a few specimens. In this type the middle por- 

 tion of the pygidium is much like fig. 2, but longer and somewhat shovel- 

 shaped. The figures are drawn from specimens in the Museum of Compara- 

 tive Zoology, (Cat. No. 1, 2, 3) and are 2 J times natural size. 



Of sixty-five specimens on which the posterior border of the pygi- 

 dium could be more or less clearly seen, fifteen had no spines inside 

 the great ones, thirty-seven had only the outer pair, and thirteen had 

 five spines. All but one of the specimens having no spines were small, 

 showing that this was not usually an adult character, but the speci- 

 mens in the other two groups were of all sizes, showing that the number 

 and size of the spines were not age characteristics. 



Measurements. — In the following measurements, the length is 

 taken along the axial line, from the front of the glabella to the middle 

 of the posterior border of the pygidium, and does not include the 

 spines. The width is taken at the genal angles, but does not include 

 the spines. 



