& W. Ford— Embryonic Forms of Trilobites. 269 



In this specimen we have the first indication of the fine system 

 of surface lines which ornaments so beautifully, in many instan- 

 ces, the adults. From the posterior extremity of the eye-lobe 

 there is a slender though distinct ridge running backward par- 

 allel with the inter-ocular spine, lying close alongside, to the 

 marginal edge and forming with it a continuous elevation. 

 Between this little ridge and its adjacent spine is the place of 

 the suture. In this, as in the preceding example, the inner 

 formed portions of the posterior margin wall against the inter- 

 ocular spines. These spines are here clearly seen to lie in the 

 marginal border, or rather between its still disunited outer and 

 inner portions, appearing as if occupying a little notch in that 

 border especially intended for their reception. The spines of 

 the posterior angles are only partially preserved. 



In figure 4a, which is of the natural size, and o the same 

 enlarged as before, the posterior margin is continuous or united 

 at the sutures ; but there is a little ridge running from the base 

 of the eye-lobe to the marginal edge, as in the preceding case. 

 This character is likewise shown in all perfect adult specimens. 

 The specimen is perfect excepting the posterior spines. The 

 characters are all only those of adult growth. There are three 

 pairs of glabellar furrows in advance of the neck-furrow, none 

 of which reach the median line, and in addition, as is likewise 

 true of most adult specimens, a short, fainter fourth pair in front 

 of the anterior extremities of the eye-lobes. The neck-furrow 

 is continuous all across. The spaces between the eye-lobes and 

 glabella are but moderately convex and were no longer pro- 

 vided with spinous appendages. The surface of the cheeks is 

 ornamented with exceedingly fine waved lines, best shown in 

 the concavity just within the marginal rim. t 



I have purposely dealt with figure 4, under this head, in 

 order to prepare the way for a better understanding of certain 

 features of structure presented by the embryonic forms to 

 which I wish more particularly to refer. Eeturning to figure 

 3a:— on comparing this form with 2a, we observe the princi- 

 pal differences to consist in the relatively greater lateral expan- 

 sion of the postero-lateral portions of the shield in the former, 

 these being extended so as to give to the form a nearly semi- 

 circular outline; the relatively smaller size of the tumid spaces 

 between the eye-lobes and glabella; the relatively nearer ap- 

 proach of the posterior portions of the eye-lobes to the glabella ; 

 and in the presence of a nearly perfectly formed, though not, as 

 yet, united posterior border. With the completion of the perfect 

 union of the posterior margin at the sutures, the inter-ocular 

 s Pines appear to have been no longer of use and were altogether 

 oppressed. Figure 4 is an example of this. But m all essen- 

 tial features of structure I believe 2 and 3 to be fundamentally 



