54 Annals of the Carnegie Museum. 



A pygidium from Trenton Falls has the same size and proportions as the 

 one from the Black River just described. 



The smallest specimen retaining the cephalon is slightly larger, as the 

 pygidium is 3.5 mm. long. This specimen is, unfortunately, slightly im- 

 perfect, so that the length and breadth of the cephalon can not be ac- 

 curately measured, but the cephalon appears to be about i mm. longer 

 than the pygidium. The glabella is slightly convex, indistinctly outlined 

 by dorsal furrows, and there are traces of three pairs of glabellar furrows 

 and a neck-furrow. The furrows are not, however, much more strongly 

 impressed than they are on some adults. In the middle of the neck-ring 

 there is a slight swelling, suggesting a median tubercle. The eye which 

 is preserved on this specimen is very large, being one-fourth the total 

 length of the head, and situated slightly less than its own length from the 

 posterior margin. Its actual position is, therefore, about the same as in 

 the adult, but relatively it is a little further forward, its posterior margin 

 being at a distance from the border of the cephalon equal to one-fourth 

 the length of the head. An adult, with cephalon 50 mm. long, has eyes 

 7.5 mm. long, and they are situated 8 mm. from the posterior margin. 

 Genal spines were present on this specimen, but are broken off. The 

 smallest specimen rgjtaining a complete genal spine has a cephalon about 

 8 mm. long, and the genal spine is 4.5 mm. long, and very narrow. 



Twenty-one specimens of various sizes were measured, and it was 

 found that with the increase in size, the width of the axial lobe of the 

 thorax increased from one-third to one-half the total width, the maximum 

 being reached on specimens with the pygidium 20 mm. long and a total 

 length of about 57 mm. On all specimens larger than this the axial 

 lobe occupied about one-half the width. The form of the pygidium was 

 found to change from approximately semicircular in small specimens, to 

 subtriangular in large ones. Thus, in the smallest specimens the length 

 is .60 of the width, while in a specimen 55 mm. long, the length is. 81 of 

 the width. The change is a gradual one, but pygidia 1 1 mm, long ha^•e a 

 distinctly triangular form, and the sides are straightened instead of being 

 rounded as in the smaller specimens. Pygidia more than 50 mm. in 

 length seem as a rule to be about three-fourths as long as wide, although 

 exceptional specimens have been seen in which the length and breadth 

 were equal. The subtriangular form of the pygidium is one of the most 

 distinctive characters of this species. The axial lobe of the pygidium 

 flattens out rapidly as the size increases. On pygidia 11 mm. long it is 

 still distinct, but not bounded by sharp furrows as in the smallest speci- 



