Diversity in the Scutes of Cheloiiia. 33 



The asyimnetrical neurals lead us to the subject of the inter- 

 adjustment of neurals and costals, a subject of sutficient significance 

 to justify its treatment in a separate section. 



3. Adjustment of Neurals and Costals. 



In the typical carapace, neural scutes and costal scutes have an 

 alternating relation. The neural series dovetails on each side into 

 the costal series and a neural seam extends transversely from the 

 apex of each costal scute. Extra scutes may occur in different parts 

 of the costal series and often only on one side. In cases where the 

 abnormality is such as seriously to disturb the symmetry of the 

 costal series, the adjustment of the neurals to the costals might be 

 accomplished in one of several ways. 



1. The sequence of neurals might remain normal, presenting 

 some such appearance as is represented in Fig. IST, I.^ Such an 

 arrangement of neurals without regard to the costals of one side has 

 not been observed. 



2. I^eural scutes might have normal relations to costals on each 

 side (cf. Fig. IST, II). There would have to be one supernumerary 

 neural in correspondence with the supernumerary costal, and some 

 of the median scutesi would occupy oblique positions. In such a 

 carapace, the antero-posterior extent of a neural scute measured 

 from the plane of its most anterior point to that of its most posterior 

 point would be unusually great. The antero-posterior extent of ISTS 

 in the figure is indicated by the line x y. Disregarding temporarily 

 some rare and partial exceptions, this plan of adjustment is not 

 observed. 



Asymmetrical costals ofteji occur in such form as to give oppor- 

 tunity for plans something like one of the above, and these plans 

 seem to offer the simplest schemes of adjustment for neurals. Since 

 with the partial exceptions that will be mentioned later, neither of 

 those plans is observed, we may assume that these hypothetical 

 schemes do not accord with the laws of growth. 



3. Assuming that a better coordinated carapace results when, 



'In Fig. N the costal series is traced from an actual specimen (No. 151) and 

 the same series is used in all three plans. 



