& W. Ford— Embryonic Forms of Trilobites. 271 



coincident with the course of the suture. In fig. 8, which repre- 

 sents the glabella, eye-lobes, and portions of the cheeks of a 

 large individual, the whole considerably weathered, there are 

 three such fractures. Of these a coincides very nearly in relative 

 position and direction with a in fig. 5. But this coincidence ap- 

 pears to me to be only accidental. It is possible, judging from a 

 specimen in my collection showing the under side of the head 

 with the hypostoma in place, that the line c coincides with the 

 suture, but I do not feel at all satisfied of this. I have never 

 been able to ascertain to my entire satisfaction the direction of 

 the suture in front of the eye in any specimen. The shield 

 appears to have fractured as easily in one direction as another. 

 This would seem to indicate that the cheeks were very firmly 

 united at the sutures in front. This, if true, has an important 

 bearing upon what I have stated in regard to the mode of for- 

 mation of the posterior border in the embryonic stages ; for if, 

 according to my view, the posterior portions of the sutures 

 were, almost from first to last in the progress of the metamor- 

 phoses, only indicated by the structure, the spaces in the devel- 

 oping margin which they were to occupy being long left open, 

 we can readily understand whv the anterior portions of the 

 shield should have been firmly held together. The changes 

 that took place in the forward portion of the shield during the 

 period of development were, as compared with those in the 

 backward, relatively unimportant both as to kind and amount. 



This species sometimes attained an enormous size. One 

 specimen of the glabella in my collection is an inch in width 

 at its narrowest part. The surface of the head in some cases 

 presents an exceedingly ornate appearance. Occasionally the 

 whole surface of the cheeks is covered with small polygonal 

 areas having a little prominence in the center of each, these 

 areas being formed by the irregular interlacing of the fine 

 raised surface lines. In other cases this arrangement holds 

 good for the more central portions of the cheeks, while toward 

 the margin the lines become more regular in direction and 

 better defined, and sometimes become so widely separated as 

 to enclose two or more rows of what were before the little 

 tubercles or prominences in the polygonal spaces. Figs. Wa-b 

 are intended to illustrati what is "here meant, b being a small 

 segment of the outer portion of a cheek greatly enlarged, and a 

 an enlargement of one of the inner portions. At other times 

 the whole surface of the cheeks is covered with tubercles with 

 n o enclosing lines, and at still others with only irregular lines 

 with no tubercles. 



Fig. 9 is of a portion of one of the cheeks preserving its 

 spine. The upper crust is nearly all removed, but at the spine 

 we see it present with its usual ornamentation. The specimen 



