& 



THE FAUNA 0-F THE ST. JOHN GEOUl'. 117 



Shield 2x4 mm. {Fig. 31.) — Thi.s is widely différent from the adult in many re- 

 spects ; the cheeks are unusually tumid, and the glabella and anterior marginal fold, 

 especially the latter, already possess the form peculiar to this species. 



The anterior margin inclines to be straight across the front and angulated at the lateral 

 third. The fold is thick and wide in front, but fades away near the front of the lateral 

 third, where the tumid cheek is devoid of protecting rim ; the triangular enlargement at 

 the front is not so marked as in the adult, owing to the furrow being less decidedly im- 

 pressed. The genal spines are about half as long as iha posterior diameter of the shield, 

 and are distinctly incurved at the points. 



The posterior margin (in the only example known) ajipears to extend into a mantle, or 

 membrane, which connects the points of the genal spines with a central spine having 

 a mesian furrow ; this mantle arches forward between the three spines, and is of 

 great tenuity ; but is bounded posteriorly by a delicate, though distinct, thickened mar- 

 gin ; no posterior marginal fold is visible, but a small tubercular elevation marks the 

 point where the posterior fold in the adult is angulated ; though the fold is absent, the 

 posterior marginal furrow is distinctly impressed ; it arches forward in the inner two- 

 thirds, and outward in the outer third. 



The glabella at this stage appears cylindrical rather than conical, owing to the high 

 relief of the anterior end, and the wide depression which at the posterior end separates it 

 from the cheeks. The first pair of furrows is sufficiently distinct ; they are directed back- 

 ward at a sharp angle, and reach the posterior slope of the glabella, but do not connect 

 with each other ; the second pair can be detected, but they are very faint ; their direction 

 is nearly parallel to the first. 



The cheeks are quite tumid in the middle and at the anterior end, but are flattened at 

 the posterior inner angle, and, in a less degree, toward the outer angle. The ocular ridge 

 is distinct ; it begins on the inner slope of the cheek, opposite the point where the dorsal 

 furrow begins to bend around toward the front of the glabella ; at this point there is a 

 small, sharp, lenticular elevation, from which the ridge arches forward across the front 

 slope of the cheek, and descends, arching backward, along the outer slope until it is lost 

 in the anterior marginal furrow, somewhat behind the middle of the lateral third of the 

 anterior margin. 



The occipital ring is well marked and prominent ; it pi-ojects behind the line of the 

 posterior margin, and is crowned by a distinct, though not a long, spine. The outer third 

 of the occipital furrow is heavily impressed, but does not extend far enough to sever the 

 ring from the cheek, there being a narrow connecting ridge. 



Sculpture. — All projecting parts of the shield are finely but, when viewed with a lens, 

 distinctly granulated, and a few tubercles are visible. 



Shield 4J X 8 mm. (Fig. 32.) — At this age the shield has a much iiearer resemblance, 

 in general aspect, to the adult. The front margin is more distinctly arched, and the cheeks 

 have the rhomboidal form of those of the full-grown animal. The facial suture can at 

 this stage be detected ; it cuts the anterior marginal fold as far from the posterior angle as 

 that angle is from the occii^ital ring. The posterior marginal fold is thin but distinct. 



Sculpture. — Not only is the surface granulated, but the tubercles are distinct and are 

 arranged as described in the account of the sculpture of the adult. A double row of 



