An apparently reasonable hypothesis is that the costal plates ankylose distally with the ribs early enough in embryonic life so that any differential in growth rate between them and the vertebral column is translated into abnormal contortions of the body. Agassiz and others have shown that the costal plates normally do not fuse with the ribs by the time of hatching; the fusion then does not normally occur in the embryonic stage. Presumably, once fused, the costal plates and vertebral column normally have equal growth rates, since the height/width ratio does not change significantly with increased size. It is well known that fusion takes place in young specimens soon after hatching; in all skeletons examined of this genus, from the smallest (62 mm. in length) to the largest (295 mm.), the fusion has occurred. Therefore, the normal time of fusion must be approximately at the time of hatching. Although costal plates and the vertebral column grow in direct proportion to each other throughout life from a period shortly after hatching, the vertebral column apparently grows more rapidly than the costals shortly before and possibly also shortly after hatching, at least in kyphotic and probably also in normal specimens. An exceptionally early date of fusion of costal plates and ribs would thus result in a kyphotic condition, and it may well be assumed that the earlier the fusion, the greater the hypertrophy would be. Whether or not this hypothesis correctly accounts for kyphosis in turtles can be ascertained only by further study. Stejneger (Bull. Mus. Comp. Zoöl., 94: 12, 1944) regards the presence of 8 neurals as opposed to 7 as an important peculiarity of _A. mutica_. The 42 specimens for which the number of neurals is recorded reveals, however, that there is greater variation than previously supposed: in 16 _A. mutica_ more than half (9) have 7 neurals and the remainder (7) have 8. Eight neurals were recorded also in 2 of 18 _spinifera_, and in 1 of 7 _A. emoryi_. Seven neurals are present in the single specimen of _A. ferox_ examined. It is of interest also that the number of costals, which has been reported to be consistently 7 in New World species and 8 in Old World species, varies markedly. In New World specimens, one _A. mutica_ has 7 on one side, 8 on the other, and 8 occur on both sides of one other (of a total of 16 examined). One of twenty _A. spinifera_, and one of eight _A. emoryi_ have 8; the single _A. ferox_ (Schneider) has 7. Accordingly the suggestion by H. M. Smith (Field Mus. Nat. Hist. Zoöl. Ser., 23:19, 1939) that _Platypeltis_ Baur be resurrected for the American soft-shelled turtles on the basis of the occurrence of only 7 costals, is untenable.