THE FAUNA OF THE ST. JOHN GROUP. 157 



the outline of the head has taken place ; this is probably due to the separation of the 

 movable cheeks. At this stage, the ocular fillet and lobe are much nearer to the anterior 

 end of the body than in later stages of growth, and the facial suture only slighty indents 

 the outline of the middle piece of the head-shield. 



4tli Stage (Plate II, fig. 3 k). — This differs from the preceding stage chiefly in the 

 enlargement of the front limb of the fixed cheek accompanying the change in the position of 

 the eyelobes, and in the advance forward of the outer ends of the posterior furrow of the 

 glabella, shewing a development of the organs beneath the shield which are connected 

 with the lobes behind the furrow. The occipital ring is still triangular and comparatively 

 flat, and the tubercle near the back edge. 



5th Stage (Plate II, fig. 3 / ). — At this period the larva assumes many characteristics 

 of the mature form of the shield, so that the species characters stand out with some distinct- 

 ness. In the form of the occipital ring there is a decided change : this now assumes the 

 compact form observable in the adult trilobite, the tubercle is placed about the middle of 

 its width, and the back of the ring rounded downward. But there remain many minor 

 point of difference between this shield and that of the full-grown trilobite : the glabella is 

 still nearly cylindrical and comes nearer to the anterior margin ; the glabellar furrows also 

 have not quite reached their normal position at maturity, but are more evenly spaced 

 along the sides of the glabella than in the adult test. 



Later stages of this trilobite are not figured or described, because the head diff'ers but 

 little in form from that of the full-grown trilobite. 



A comparison of the two series of embryonic and larval forms on Plate I, with those 

 on Plate II, will at once shew a close genetic relation between Agravilos (as exhibited in 

 its earliest types) and Liostracus on the one hand, and Ptychoparia and Solenopleura on 

 the other ; it will be observed that the embryonic forms in Plate I possess a much larger 

 glabellar ridge than those of Plate II ; the two sets also differ in the plainness of the 

 furrows, which are scarcely distinguishable on the embryonic forms of Agraulos and Lios- 

 tracus, but are plainly visible on those of Ptychoparia and Solenopleura. In regard to 

 the two latter genera, a difference in the rate of development of the embryonic and larval 

 forms may be observed, for if the heads of corresponding size in the two series on Plate II, 

 be compared, it may be seen that those of Solenopleura develop the adult characters with 

 the greatest rapidity. The difference between these two series may, however, be inter- 

 preted in another way, namely that growth was more rapid in Ptychoparia, and that this 

 species was larger at the corresponding stages than the species of Solenopleura figured. 



Solenopleura Acadica (Plate II, figs. 5 n, b and 6.) 



Solenoplevru Acadica, Whiteaves MSS., Trans. Roy. Soc. Canada, Vol. Ill, Sec. IV, p. 76, 

 PI. VII, fig. 15. 



This compact little species was described in Mr. Whiteaves's words in a former 

 volume of these Transactions, but its characters were there very briefly presented ; and as 

 for the purposes of this article, a fuller presentation of its features becomes necessary, the 

 following additional description is given : — 



The outline of the body is oval, with short points at the genal angles. 



