THE FAUNA OF THE ST. JOHN GEOUP. 113 



first and second ribs toward the anterior side of the outer end of these ribs. The first 

 furrow both of the axis and marginal third are strongly impressed. The half rib connects 

 at the outer angle of the pygidium with a distinct border, which at the inner edge is 

 bounded by a sharp and straight, thread-like ridge. 



Sculpture. — Both the inner and outer surfaces of the test in this pygidium appear 

 smooth, but with a lens the outer surface may be seen to be very finely granulated. 



[N. B. — There is a broad variety of this pygidium (^ X 18, or 8 X 20 mm.) with more 

 distinct ribs both on the axis and lateral lobes, and having a more quadrate end to the axis.] 



Varieties. {Figs. 23 and 2Zb.) 



Var. a. arcuata. The distinctive features of this form are not very easily pointed out, 

 but are sufficiently obvious on comparison of a number of individuals ; the difference from 

 the type is most conspicuous among the larger tests, and it is not so easily recognized 

 among very small ones. It is quite possible that it may be only a sexual variation of 

 form. This variety differs from the type in having a more conical glabella, rounded 

 rather than squared in front, and having flattened slopes on each side of the axis. The 

 anterior marginal fold is more strongly arched forward in the middle, is wider and has a 

 longer slope to the furrow than the corresponding fold in the typical form ; the furrow 

 also is wider. The inner end of the ocular ridge is more prominent and rounder in this 

 variety than in the type. 



Development of the YouNa. 



The heads of this species have been found from the length of two millimetres upward. 



Shield 2x3 mm. {Fig. 25.) — In this species the earliest known stage resembles the 

 adult much more nearly than the youngest, in the preceding species, does the mature indi- 

 vidual ; and yet it presents important differences. 



The glabella is cylindrical and about two-thirds of the whole length of the shield, 

 whereas in the adult it is not much more than one-half. But the disparity in width is 

 greater, for at this stage the glabella is only one-fifth of the width of the buckler, while in 

 the larger tests it is about one-third Only the posterior pair of fiirrows can be detected at 

 this age, and they are more strongly directed backward than those of the adult. 



The occipital ring is peculiar and quite different from that of the adult. It is trian- 

 gular in outline, with the spine set well back on the axis ; the two anterior angles merge 

 into the corners of the cheeks at the point where, in the preceding species and in C. elegans, 

 a small spine is situated. 



The posterior marginal fold is well defined, but sharp and narrow, and the genal spines 

 are also narrow and about as long as one-third of the posterior diameter of the shield. 



The cylindrical glabella at this stage recalls the form common in the genus Ptychoparia, 

 and when the ocular ridge peculiar to the Conocoryphea is obscure, it is sometimes diffi- 

 cult to distinguish the young of the Ptychoparians from the fry of this sj^ecies. 



Shield 3 X 4J mm. {Fig. 26.) — At this period the glabella shows much greater lateral 

 volume, but only one pair of furrows is yet visible. The anterior marginal fold is now 



Sec. IV., 1884. 15. 



