64 Eobert E. Coker. 



rectangular (Fig. 79). Such a shield was found in two of four 

 shells of Colpochelys kempii, and in three of thirty-one specimens of 

 Chelone mydas. In several cases there is a horizontal seam confined 

 to one side, which, if complete, would isolate such a scute. (Nos. 

 3, 54, 103, 114, 120, Figs. 75, 96, 101 and 102.) Hence the seam 

 which cuts off this scute from !N^5 occurs more frequently on one side 

 than on both. The partial independence in variation shown by the 

 two sides is nicely illustrated in this case, as in many others. 



A seam may start from the usual point of origin of the seam 

 bounding the rectangular scute posteriorly, but bend posteriorly to 

 meet the last costal distally or the marginals (Nos. 37, 73, 109, 196). 

 Cf. Figs. 71, 99, and 81. A short seam completed by a fiirrow is 

 sho\\ai in Figs. 55 and 85. 



In Fig. 96 N4 is partly divided by a seam on the right side which 

 is completed on the left l)y a furrow. 



Adjustment of Ncurals and Costals. 



JSTo. 81 (Fig. 77) illustrates well this adjustment: neurals and 

 costals in alternating relation. In this specimen I infer that LCI 

 is wanting and that a pair of supernumerary scutes is present ; but, 

 disregarding this inference, there are, on each side, five costals in 

 contact with neurals posterior to ISTl, while in the normal carapace 

 there are only four. As the costals are nearly symmetrical from this 

 point posteriorly, a single nearly symmetrical extra neural serves 

 to maintain the usual alternating relation. In 31 (Fig. 65) the 

 supernumerary costal is on the left side alone, and here again we 

 find on each side the usual a'lternating relation of neurals and costals. 

 Counting down the left side there are six neurals, counting down the 

 right side there are five. The supernumerary neural is not abso- 

 lutely restricted to the left side nor unmodified on that side, but 

 tapers to a point. It seems that in Thalassoclielys, the asymmetrical 

 scutes' are not in general as much restricted to one side as in Malac- 

 lemtvmys and Graptemys. Shells, such as l^^o. 94 (Fig. 90) and 97 

 (Fig. 91) are very suggestive of the types noted in the first part of 

 this paper (p. 34, above), but more commonly the asymmetrical ele- 

 ments are of the type of those of 43 (Fig. 72) and 90 (Fig. 86),— 



