"ABNORMALITIES" IN CHELONIA. 81 



A,B, C, D and E, beginning at the anterior end. Two specimens 

 of Chrysemys marginata have extra scutes at A (Fig. 47). Two 

 specimens of Chclydra have extra scutes at C (Fig. 45). One 

 specimen of Graptcuiys has extra scute at D (Fig. 48). One 

 specimen of Graptcmys has a pair of extra scutes at E (Fig. 46). 

 The primitive condition was probably one in which a scute was 

 present at each point of union of four plastron scutes, but the 

 fact that even in the tail of Chelydra this row is either partially 

 or wholly wanting indicates the rather uncertain character of 

 the row. In the specimens listed above scutes are found occur- 

 ring in four places out of a possible five. No doubt a larger col- 

 lection would serve to fill in this gap. 



I consider these recurrences as true reversions to ancestral 

 conditions ; and that they come under the head of systematic 

 atavism I see no reason to doubt. 



How the typical number of scute rows seen in our modern 

 tortoises has been acquired has, perhaps, been sufficiently dis- 

 cussed and it now seems necessary to consider the processes 

 that have brought about the reduction of the number of scutes 

 in a row for it is beyond dispute that such a reduction has 

 taken place. 



3. Supernumerary Scutes in a Row on the Carapace. 



The literature on this subject is limited to one paper, Gadow's 

 much-discussed " Orthogenetic Variation in the Scutes of Che- 

 Ionia," that was published in Willey's Zoological Results in 

 1899. The author gives a very interesting account of the con- 

 ditions found in the common loggerhead turtle, Tlialassoclielys 

 carctta. He has gathered together a miscellaneous assortment 

 of some sixty-nine specimens of various sizes, principally new- 

 born, from many parts of the world. On the basis of this col- 

 lection he comes to the conclusion that scute reduction proceeds 

 along certain definite lines. His observations, however, are 

 limited to reductions in the neural and costal rows. According 



t> 



to Gadow, the ideal ancestral condition is one in which the neural 

 and costal bony plates determine the number of scutes. The 

 author's idea is that there was originally a scute for each of these 

 plates. 



