IO2 H. H. NEWMAN. 



lateral and two ventral, and have named them after their homo- 

 logues in the carapace and plastron, neurals (TV), costals (C), 

 marginals (M), and plastrals (P). Smaller and less regular rows 

 of tubercles and scales are homologized with the secondary and 

 lost rows as follows : inframarginals (JM\ supramarginals (SAT), 

 interplastrals (IP], and neuro-costals (NC\ a row not occurring 

 in any modern species. In the following more detailed descrip- 

 tion the above names will be used for the rows concerned. 



The costals are large scutes with a marked tendency toward 

 dorsi-ventral flattening. Their apices are directed posteriorly 

 and the growing point is thus situated near the posterior margin 

 of the scute, a fact that causes the scute to grow anteriorly and 

 laterally and very little posteriorly. The marginals are as large 

 as the costals and are flattened so as to expose two surfaces, a 

 dorsal and a ventral. The apex is directed away from the axis 

 of the body and the growth of the scute is principally inward and 

 slightly forward. The plastrals are paired, large, flat and nearly 

 rectangular. They suggest by their appearance the plastron 

 scutes. Their growth is inward and forward, as is that of their 

 homologues in the plastron. 



The two secondary dorsal rows, supramarginals and neuro- 

 costals, consist anteriorly of small tubercles that fade out pos- 

 teriorly into small irregular flat scales. The inframarginals are 

 not tubercular but are rather large and fairly regular flat scutes. 

 The interplastrals are diamond-shaped scutes forming a discon- 

 tinuous row and occupying the angles made by four plastrals. 

 This row is sometimes entirely absent on the tail-trunk of 

 Chelydra. 



It is clear then that in the tail and tail-trunk of Chelydra we 

 have seven principal rows of scutes and between each pair of 

 principal rows a secondary row, less regular and far less promi- 

 nent. This would give a total of fourteen rows, of which seven 

 are primary and seven secondary. Supposing that the carapace 

 was originally continuous with the tail-trunk, we must imagine a 

 gradual suppression of the secondary rows by the primary ones, 

 until in the most highly specialized condition, seen in the terres- 

 trial Emydidae, only the seven primary rows have survived. 

 The order of loss of the seven secondary rows may be con- 



