ORTHOGENETIC VARIATION IN THE SHELLS OF CHELONIA. 219 



There can be no doubt of the former existence of Chelonians which normally 

 possessed eight median and eight pairs of costal epidermal scutes, and that these 

 corresponded with the usual number of eight transverse series of dermal bony plates. 

 As many as 11 median dermal bones are known in some fossil forms with as many 

 as 10 pairs of costal plates, corresponding with 10 rib-bearing dorsal vertebrae. In 

 Chelydra serpentina there are not less than 12 median plates, including the last un- 

 paired " marginal." 



A reduction from plated to free, unarmoured dorsal vertebrae has taken place, 

 and does still take place, at the root of the neck, and behind the sacrum at the 

 root of the tail. Since there are from 11 to 12 to 13 to 14 pairs of marginal scutes, 

 it is reasonable to assume 14 as the highest indicated number of metameres which 

 have entered into the composition of the dorsal shell. Those early tortoises must have 

 been more elongated and far less broad than any known Chelonian. 



Moreover, the recent Macroclemmys temndncki possesses three to four so-called supra- 

 marginal scutes (analogous to the infra marginals of Chelydridae and Dermatemydidae) 

 which indicate the original typical number of longitudinal scutes, seven in all, on the 

 back, namely, an unpaired median, a pair of costal, a pair of supramarginal, and a pair 

 of marginal elements. There can also be little doubt but that the median series was 

 originally double or paired ; cf. specimens, numbers 7 and 43, and the analogy with 

 Crocodiles. 



The so-called first pair of marginal plates of Dermatemydidae and Cinosternidae 

 has underlying a pair of rib-like processes, and the fossil Chelydropsis has two suc- 

 cessive "nuchal plates." 



All recent Chelonians possess eight free cervical vertebrae and ten dorsal vertebrae, 

 including the sacrals, but the fusion of the vertebrae and ribs with the carapace is 

 restricted to eight vertebrae and ribs, the 1st dorsal (9th of the whole series) and 

 the 10th dorsal (18th of the whole series) possessing each only a very thin and small 

 pair of ribs which do not enter into the formation of the carapace by fusion with a 

 pair of costal plates. 



Then follow several, from one to three median dermal plates, the so-called pygal 

 plates, without corresponding lateral or costal plates. The first pygal belongs to the 

 last dorsal or 18th vertebra. The last pygal cannot be distinguished from a pair of 

 fused marginals. 



This is the arrangement of the majority of Chelonians, but in the Genera Cistudo, 

 Cinosternum, Dermatemys and Staurotypus the actual connexion of the carapace with 

 the vertebral column is now restricted to the 10th to 14th vertebrae. 



I therefore conclude that at an early ancestral stage, not necessarily that of the 

 primordial Chelonian, the plates and scutes of the back were arranged as follows : 



All the metameres carried originally a series of transversely arranged dermal plates 

 and scutes, which in the region of the trunk, according to the greater bulk of the 

 body, increased in size, converging towards the root of the neck and upon the tail. 



About 14 metameres were distinguished by the greater size of the dermal plates, 

 each transverse series consisting of a median or neural and three pairs of lateral 

 elements, in all eight. The median pair fused into an unpaired neural. The next 



