3-i Eobert E. Coker. 



(a) on each side there is practically the usual relation between 

 costals and neurals, and (bj neural scutes have not an antero-pos- 

 terior extent that is relatively unusually great, we may imagine 

 such a plan of neurals as is represented in Fig. jST, III. On each 

 side neural and costal scutes alternate in position, and the two 

 series dovetail together in usual fashion, while cross seams so divide 

 the neural area that no individual scute has an excessive antero- 

 posterior extent. This plan is not a hypothetical one, as were the 

 others, but is an inference from the observations. Compare with it 

 the plan of scutes illustrated by Xos. 151 (Figs. 7 and 15), and 

 195 (Fig. 8). 



The first two types of adjustment assumed above (hypothetical) 

 may thus be defined: 



1. Neurals with normal adjustment to costals on one side, but 

 not on the other. Individual scutes show this plan very rarely. 



2. jSTeurals with normal adjustment to costals on both sides, 

 but with the two sides of the neural series related in a way that is 

 rarely observed. 



The third type (observed) may be defined thus: 



3. Neurals with normal adjustment to costals on each side ; 

 the two sides of the neural series unsvmmetrical and so related to 

 each other that the plan of each side is largely restricted to that 

 side. 



In this manner of gTowth, then, we have a neural plan (or lialf- 

 plan) on one side, a different neural plan on the opposite side, and 

 each plan is adapted to the costal plan of the same side (cf. the 

 diagram, Fig. O, III a. The mesial region of the carapace is 

 supposed to he covered hy a strip of paper). The plan of each side 

 does not usually terminate abruptly in the median line as repre- 

 sented in the diagram, III, b (cf. however, ISFewmann's Fig. 6),* 

 but the scutes necessary to the plan of one side extend a little 

 over the median line (III, c), sometimes almost or quite across 

 to the opposite costals (III, d). In consequence, we get, not a 

 plan on one side independent of the plan on the other side, but on 

 each side a plan that is mora or less modified hy the over-extension 

 of the scutes necessanj to iJw, plan of fJi.r opposite side. 



*Fig. P, XTI. 



