268 S. W. Ford—Embryonic Forms of Irilobites. 



facial suture in the adult. At this stage that portion of the 

 posterior margin of adults which lies within the sutures is not 

 even indicated. The specimen presenting these remarkable 

 features of structure is in a beautiful state of preservation. 



Fig. 2a represents another specimen, natural size, and 2b the 

 same enlarged three and one half diameters. The lower left 

 hand portion of the border is slightly restored in each. This 

 specimen is, in extreme width, two-thirds larger than the first 

 example, and in extreme length, exclusive of the spines, one- 

 half larger. The form has changed considerably, but it re- 

 quires but a moment's attention to convince one that the fun- 

 damental structure is, in all respects, the same with that of the 

 preceding example. The shield is more spread out laterally, 

 and markedly so at the mid-length ; the eye-lobes in their 

 ^forward portions are more arched ; the glabella is more acutely 

 and regularly tapering ; and the proportional distance between 

 the inter-ocular spines at their origin is considerably increased. 

 The glabella is marked by four distinct furrows extending all 

 across as before. Immediately in advance of the fourth or 

 foremost furrow there is a slight ridge extending all across the 

 anterior lobe, which gives to the eye-lobes the appearance of 

 meeting on the median line of the glabella. The neck-furrow 

 is strongly marked. On either side of the neck-segment is a 

 short, deep, longitudinal furrow with an elevated rim bounding 

 it behind. We have here rudely sketched out that portion of 

 the posterior margin of adults which lies inside of the sutures. 

 In contact with this margin there is a somewhat convex, trian- 

 gular body, the central portion of which is damaged, but which 

 shows on each side two or three obscure furrows extending 

 outward and backward. This body, both from what Barrande 

 has taught respecting the mode of development of the thorax 

 of trilobites and from the appearances themselves, I consider 

 to be most probably a rudimentary thorax and pygidium com- 

 bined. From the even manner in which it is outlined it is 

 probable that none of the segments were yet free. The surface 

 of both this and the foregoing specimen is smooth. 



Now, if we examine the figures it will appear evident, I 

 think, that, between 1 and 2, one or more intermediate forms 

 may exist, and between 2 and 3 a considerable number of such 

 forms. As yet, however, no such forms have been obtained. 

 Figure 3a shows, natural size, and 36 twice enlarged, a head 

 whose development is evidently nearly completed, but which 

 is, nevertheless, embryonic in certain of its features. There 

 rt the strong inter-ocular spines and the glabellar ridge 

 uniting the eye-lobes in front. The glabellar furrows also extend 

 all across, but become fainter on the median line. The eye-lobes 

 are rather more regularly curved than represented in the figure. 



