"ABNORMALITIES IN CHELONIA. Ill 



show a fairly complete set of ocellated spots, but others show 

 almost colorless scutes. Adult specimens seldom show any sign 

 of pigmentation on the plastron. The markings then are largely 

 juvenile in character and are subject to very great individual vari- 

 ation that tends toward the total obliteration of the original color 

 pattern. An examination of a number of specimens, however, 

 shows that each of the plastron scutes may have its ocellated 

 spot. 



The most confusing markings of all are those that occupy the 

 median portion of the plastron in juvenile specimens. A remark- 

 able secondary complexity of pattern has arisen in this region 

 that would be almost impossible of solution were it not for the 

 close series of stages leading up from the simplest conditions. 

 The simplest form of marking consists of small diamond-shaped 

 patches of pigment at the inner angles of the plastron scutes. 

 These spread along the margins of the scutes, form bands by 

 splitting, and finally produce the complex lyriform pattern that 

 one finds quite frequently. The position of the simplest marking 

 is identical with that of the hypothetical interplastral scutes and 

 probably once constituted the color-marking of this row of scutes. 



All of the carapace and plastron markings have thus been 

 accounted for as the growth centers of existing or lost scutes. 

 This has been done in a species with a higftly intricate color pat- 

 tern and could be applied successfully, I believe, to any other 

 species. 



It should be mentioned that the color pattern of Graptemys 

 reaches its highest development in specimens of the first year. 

 This is the time when protective coloration is a necessity, as the 

 carapace is not sufficiently Ossified to furnish a protection. Old 

 specimens retain scarcely a trace of the original pattern, only a 

 very faint reticulation being visible. 



It might be suggested that the ocellated spots of the Triony- 

 chidse are vestiges of scutes long since lost. The general number 

 and arrangement of these spots tends to bear out this suggestion. 



SUMMARY. 



Palaeontological and embryological evidence is at variance as 

 to the origin and character of the neural and costal plates, but 



