86 



A REVISION OF THE COTYLOSAURIA OF NORTH AMERICA 



the turtles the squamosal extends backwards overlying and surrounding the upper 

 end of the quadrate. This is exactly the position of the squamosal or squamosal+ 

 tabulare in the Diadectidce: the direct fusion of squamosal and tabulare would 

 produce the condition of the testudinate squamosal 



Diadectes molaris Cope .... (p. 15) Diadectes latibuccatus Cope ... (p. 19) 



Diadectes biculminatus Cope . . (p. 17) Diadectes maximus Case .... (p. 20) 



Diadectes fissus Cope (p. 17) Diasparactus zenos Case .... (p. 21) 



These species are all so incompletely known that an extended morphological 

 discussion is impossible. The known facts are set forth in the systematic discussion. 



Chilonyx rapidens Cope. (Plate 10, fig. 2.) 



Characteristic specimen: The type, No. 4357 Am. Mus. Nat. Hist. Cope Coll. 



This genus probably held the same relation to Diadectes that Geikia did to Pareia- 



saurus, Miolania to other pleurodrian turtles, or that Phrynosoma does to related 



lizards. The development of bosses and bony horns is probably an expression of 



pmx ~\ 



yiijiiJ". /■<!i)j!jf \ 



Fig. 32. 



A. Upper Burfacc of skull of Chilonyx rapidens. Sutures according to the author. X J. No. 4357 Am. Mus. n, nasal; 



mx, maxillary; /, frontal; pf, prefrontal; ptf, postfrontal; po, postorbital; p, parietal; sq, squamosal; qj, quadratojugal; 

 ep, tabulare; so, supraoccipital plates. 



B. Restoration of skull of Chilonyx, upper view. After Cope, pmx, premaxillary; n, nasal; mx, maxillary; ptf, prefrontal; 



pof, postfrontal; /, frontal; /', jugal; z, zygomatic; p, parietal; sm, squamosal; s/,prosquamosal; int, tabulare. 

 C Restoration of skull of Chilonyx, lateral Tiew. After Cope, pmx, premaxillary; mx, maxillary; n, nasal; /, lachrymal; 

 pef, prefrontal; /, frontal; pob, postorbital; pof, postfrontal: sm, squamosal; si, prosquamosal; int, tabulare; x, zygo- 

 matic! q, quadrate. 



the same force which led to the development of spines in Naosaurus and Dimetro- 

 don^nd to similar extravagances in the Amphibia of the same beds. We have here 

 a glimpse of overspecialization among the Diadectids, perhaps presaging their 

 extinction. 



In the original description of the genus (25) Cope writes as follows: 

 "The condition of the specimen is such that the composition of the skull may 

 be readily made out. The postfrontal bones are large, and form the superior border 

 of the orbit. At the front of the orbit they reach the prefrontal, thus excluding the 

 frontal. The parietal bones are wider than the frontals, and are bounded laterally 

 by the postfrontals and the squamosals and by an element between the squamosal 

 and exoccipital, which occupies the position of the intercalare of the Stegocephah. 

 Below this bone, on the inner side of the suspensorium, is the probable prootic. 

 The squamosal, or an element which I can not distinguish from that bone, extends 

 to the condyle of the quadrate, concealing that bone from external view. The 



