Diversity iii the Scutes of Cheloiiia. 29 



are most iustructive. In Fig. S, PL III, there are at least seven 

 elements in the neural series, of which the most posterior five fit 

 in together as if crowded out of a condition of longitudinal sequence, 

 each scute extending across the median line. It will be noted that 

 while the rings of gro\vth indicate that each scute has gTown most 

 in the antero-external direction (toward the antero-lateral costal), 

 jet each scute shows comparatively broad rings toward its antero- 

 mesial neighbor of the same series, although growth in this direc- 

 tion must cause further distortion of the keel by pushing the prom- 

 inences further and further away from the median line. Neverthe- 

 less the prominences still make a continuous but crooked carapace 

 keel. No. 151 (Fig. 15) presents a different condition. Numbering 

 the scutes from Nl posteriorly, (cf. PI. IV, Fig. 7, of the same cara- 

 pace) N3 and N5 are almost parallel in position to Nl, and the keel 

 of N5 extends anteriorly half-way by the keel of N4, to which it is 

 exactly parallel. Clearly, in this case the keel of the carapace 

 branches and is double for a part of its course. 



No. 150 (PI. IV, Fig. 0) may offer a clear case of longitudinal 

 sequence, though the supernumerar}^ scute is largely on the left side. 

 No. 139 had a very similar anomalous element, but entirely to the 

 left of the median line and without the keel prominence. 



Nos. 167 (PI. VII, Fig. IG) and 251 (PL VII, Fig. 14) are com- 

 parable to the first illustration given above. 



In the cases cited, which are representative of a number of others, 

 the keel prominences throw no definite light on the question at hand. 

 Is it to be assumed that such scutes are primarily in longitudinal 

 sequence but are crowded out of position? Or that they are really 

 paired scutes, asymmetrically placed ? Or is their significance some- 

 thing still different ? Newmann has decided in favor of the first 

 explanation, but suggests that Gadow would probably consider such 

 types as evidence of the original paired character of the neural row. 



It is possible hypotheficaUy to regard many of the asymmet- 

 rical neural anomalies as illustrating only a secondary asymmetry, 

 a modified longitudinal sequence; but some cases can hardly be 

 referred to such a condition, especially when a portion of the keel 

 parallels another portion, as in No. 151 described above. Now, is 



