"ABNORMALITIES IN CHELONIA. 77 



send out branching processes that gradually displace the dermal 

 connective tissue of the tubercles and fill the space with bone. 

 Complete ossification of these tubercles does not occur until the 

 animals are several years of age. 



III. DESCRIPTION AND DISCUSSION OF ABNORMALITIES. 



That a process of reduction both in the number of rows of 

 scutes and in the number of scutes in surviving rows has taken 

 place seems highly probable. From this standpoint I made a 

 systematic study of all the abnormal specimens that showed traces 

 of these lost rows or lost scutes. Inframarginals of all grades of 

 prominence were found in specimens of Graptcmys geograpkica 

 and Chrysemys marginata, while interplastrals were found more 

 rarely in the same two species. It will be noted that both of 

 these recurring rows are plastron rows which probably means 

 that the carapace has reached a high degree of fixity with refer- 

 ence to number of rows. Yet many abnormalities are found that 

 indicate that the reduction in the number of scutes in a row was 

 of comparatively recent occurrence. 



These abnormalities will be discussed under three heads : (i) 

 Inframarginals, (2) interplastrals, (3) supernumerary scutes in a 



row. 



i . Inframarginals. 



The occurrence or non-occurrence of inframarginals has formed 

 the basis for separating the Thecophora into two great groups. 

 Gadow in his volume on Amphibia and Reptiles gives Boulanger's 

 key for classifying Chelonia. In this the two groups are charac- 

 terized as follows : 



1. Pectoral shields separated from the marginals by inframar- 

 ginals Chelydridae, Platysternidae, Cinosternidae. 



2. Pectoral shields in contact with the marginals Testudini- 

 dae, Chelydidas, Pelomedusidae. 



It is evident that the more primitive families possess as normal 

 factors this row of scutes while the more specialized families 

 normally lack this row. When, however, dozens of specimens 

 of Graptcmys and Chrysemys possess this row in more or less 

 perfect form, I am forced to consider this phenomenon as a well- 

 marked case of systematic atavism. In view of the fact that no 



