352 PALEONTOLOGY OF OHIO. 



the Illinois coal-field/:^ is a still more stout and squat form. In Ptyonius 

 and Oestocephalus we have very elongate types, but in Phlegethontia, and 

 probably MolgopMs, we have veritable Batrachian snakes, the Phlegethon- 

 tia linearis resembling a whip-snake in its proportions. In some of the 

 Labyrinthodonts of the Old World we see forms rivaling the Saurians 

 in power of armature and protective shields. All of these forms have 

 been found to be referable to the Batrachia in their agreement in the 

 following characters : 



Basis-cranii composed of a parasphenoid bone, which is developed in 

 the periosteal membrane ; two occipital condyles, one on each exoccipital 

 bone J a large squamosal bone, covering the quadrate externally; stapes 

 external; articular bone present ; the atlas undivided, with single cen- 

 trum; coracoid distinct from scapula; limbs, when present, composed of 

 femur, ulna, and radius, and pes or manus. 



There are six orders of this class, which do not differ so much in exter- 

 nal features as the orders of Reptilia, but are, in composition of skeleton, 

 circulatory system, etc., quite as distinct. These were fully defined by 

 the writer in 1868,t but farther investigation has added other important 

 characters, especially with regard to the composition of the posterior walls 

 of the cranium. The orders may be separated in key form, as follows : 



I. Supraoccipital and epiotic bones present ; supratemporal bone roof- 

 ing temporal fossa. Stegocephali. 



II. No supraoccipital, epiotic present ; a palatine arch, no temporal 

 roof. Proteida. 



III. No supraoccipital nor epiotic bones; frontal distinct; quadrate 

 osseous; no maxillary nor palatine arch. Trachystomata. 



Maxillary arch, no palatine. Urodela. 



Both maxillary and palatine arches. Gymnophiona. 



IV. No supraoccipital nor epiotic bones ; frontal and parietal united ; 

 quadrate cartilaginous; caudal vertebrae connate. Anura. 



In all Batrachians the vomer is double, and usually bears teeth ; the 

 premaxillary is single or double, and the teeth are never planted in 

 deep alveoli. There are six orders, as follows : 



TRACHYSTOMATA. 



Caudal vertebrae and frontal bones distinct ; inferior pelvic elements 

 not confluent. 

 0,0, supraoccipitalia,maxillaria, prefrontalia, palatina, and pterygoidea 



* See Geological Survey of Illinois, by A. H. Worthen, Vol. IL, p. 136. 

 t Proceedings Philad. Acad., 1868, p. 211 ; Extinct Batrachia and Keptilia of North 

 America, in Transactions Amer. Philos. Society, XIV., p. 4, 1869. 



