PRIMARY AND SECONDARY CHARACTERS. 23 



ON THE PRIMARY AND THE SECONDARY CHARACTERS OF TURTLES. 



It may be interesting and productive of some useful result to endeavor to separate 

 the osteological characters that belonged to the most primitive turtles from those 

 that may be called the secondary characters, those that have been acquired in later 

 times through the exaggerated development or reduction or the suppression of cer- 

 tain parts ot the skeleton or their modification of form and connections, in order to 

 adapt them to new uses. We will, as heretofore, consider first the shell of the turtles. 



As already stated, the writer holds the view that the earliest turtles possest 

 practically two kinds of shell, one purely dermal, consisting of probably a mosaic 

 of small bones arranged in at least 12 longitudinal zones. Each zone probably 

 consisted of a row of larger bones, bordered on each side by smaller ones. It is not 

 necessary to suppose that the spaces between the zones were wholly occupied by 

 these smaller bones. Each of these bones was covered by a horny scute. The 

 nearest approach to such a dermal shell is in our days seen in Dermochelys, as has 

 already been stated. 



Beneath the skin there seems to have existed a carapace more or less complete 

 which consisted of a nuchal, a median row of neurals, 8 pairs of costals, a pygal, 

 probably one or more suprapygals, and about 11 peripherals on each side. To 

 what extent the neurals and the costal plates had become anchylosed to the neural 

 spines and the ribs respectively it is now impossible to determine. Nor can we say 

 to what extent the various elements of this carapace had become connected with one 

 another. The existence of the dermal carapace would appear to indicate that the 

 subdermal box was not yet closed.* 



There was a subdermal plastron that was composed of at least u bones. 

 Portis has described Polysternon, which had an additional pair of bones between the 

 hypoplastra and the xiphiplastra, making 13 all together. Between these various 

 bones there may have existed more or less extensive fontanels. 



According to the author's views, as time went on the external, mosaic-like shell 

 disappeared in most turtles, while a more efficient armor was developt out of the 

 subdermal elements. In the ancestors of Dermochelys, however, the dermal armor 

 was retained, while the more deeply seated one disappeared, with the exception of 

 the nuchal bone. 



It is proper to state that all authors do not hold that Dermochelys has descended 

 to us in a direct line distinct from other sea-turtles, but has been derived from them 

 at a more recent date. On this subject the reader must consult the papers of Baur, 

 Boulenger, Case, Dollo, E. Fraas, Hay, Van Bemmelen, and Wieland. 



Most Thecophore turtles have lost the bones of the outer dermal shell. There 

 are yet traces of it perhaps in the dorsal and ventral keels of various turtles, in the 

 tubercles that diversify these keels, and especially in the rows of horny scutes that 

 had their origin from these dermal bones. In one turtle, however, Toxochelys, 

 of the Upper Cretaceous, there are yet remains of the dermal armor in the shape of 

 a row of bones along the dorsal median keel. For a description and illustrations of 

 these the reader must consult later pages of this work. 



In a paper written in 1898 (Amer. Naturalist, xxxn, p. 929) the writer denomi- 

 nated such bones as the nuchal, the peripherals, and suprapygals as "fascia bones." 

 This was done simply to distinguish them from more superficial bones, called 

 dermal. In a recent paper Mr. H. H. Newman (Biol. Bull, x, p. 74) has referred 

 to this paper and has insisted that the nuchal is a dermal bone. Such it doubtless 



*For Dr. George Baur's latest views regarding the primitive condition of the carapace and plastron 

 of the turtles see Anatomischer Anzeiger, XH, 1906, p. 567. 



