484 BULLETIN 58, UNITED STATES NATIONAL MUSEUM. 



The ]>r()]>or iiitcrrelationship between the various types of living 

 Chelonians has not been finally determined as 3'et. The greatest 

 stunibliiig ])lock has been the Dermochelys, which some authors, such 

 as Baur and Fuerbringer, regard as a highly specialized form more or 

 less closely allied to the other marine turtles, while others have taken 

 the opposite view, regarding it as one of the most primitive types and 

 so radically different from all the other chelonians as to make it expe- 

 dient to collect the latter in one great suborder and to keep Dermo- 

 chelys by itself in another. The former view is scarcely tenable, if 

 Goette* is correct in regarding the bony carapace of the majority of 

 turtles as an integral part of the skeleton as opposed to the dermal 

 origin of the dorsal mosaic of Dermochelys. On the other hand, the soft- 

 shelled turtles exhibit so many differences from those with a horny 

 covering that it seems inexpedient to include them in the same cate- 

 gory. For these reasons it has been thought best to adhere to the 

 division first proposed by Seeley, in 1882, but to substitute subordinal 

 names for those chosen then by him, these having the form of family 

 names, as understood by most zoologists. 



The characters which define the three suborders may be tabulated 

 as follow^s: 



a' No solid carapace, the vertebn* and rilis being separated from a sliell consisting of a 



mosaic of numerous small polygonal bony plates imbedded in a leathery skin; no 



descending process of the parietal bone; limbs without claws. .1. Athec^e, p. 485. 



a'- A solid carapace of a few large symnretrical bony plates not separated from the 



underlying vertebrae and ril)s; i)arietals with descending processes; limbs with at 



least one claw each. 



¥ Body covered with horny scutes arranged differently from the bony plates beneath; 



epiplastra and hyoplastra in contact, not separated by entoplastron ; center 



of last cervical and first dorsal vertebra; articulating with each other; fourth 



digit never with more than three phalanges; jaws covered by horny sheaths 



not concealed imder fleshy lips 2. Laminifer.\, p. 488. 



b^ Body covered by an undivided leathery sliin without scutes; epiplastra separated 

 by entoplastron from hyo-plastra; last cervical vertebra articulating with first 

 dorsal by zygapophyses only; fourth digit A\ith more than three phalanges; 

 jaws concealed under fleshy lips 3. Chilot.e, p. 513. 



The turtles occurring within our limits may be easily referred to 

 their resj^ective families by the following: 



ARTIFICIAL KEY TO FAMILIES OF TURTLES. 



a^ Limbs ciawless; back with live longitudinal keels or ridges (Athecie). 



Dermochelid.e, p. 485. 

 a^ Liml)s with at least one claw each; back, if keeled, with at most three longitudinal 

 ridges, 

 fe' Outer body cnvcrin^ a soil skin wilhinil horny plates (CMlolx). 



Trjonychid.^5, p. 514. 

 h'^ Outer body covering consisting of symmetrical horny plates (Lamini/era). 

 c^ Limbs not paddle-shaped ; claws four or five on each limb . . Testudinid.e, p. 488. 

 c^ Limbs paddle-shaped; claws two or one on each limb Cheloniid^, p. 506. 



"Zeitschr. Wissensch. Zool., LXVI, 1899, pp. 407-434. 



