174 THE COAL MEASURES AMPHIBIA OF NORTH AMERICA. 



The form combines in an unusual and remarkable degree reptilian and amphibian 

 characteristics. The leg bones, pelvis, and tarsus are all strikingly reptilian, but the 

 phalanges in the arrangement of elements are so typically amphibian that if we had no 

 other means of diagnosis we would incline to locate this Coal Measures species among 

 the Amphibia. The leg (plate 23, fig. i) recalls in its structure that of another Coal 

 Measures species, Eosauravus copei Williston, which is, however, clearly a reptile. 

 While there is a general degree of similarity between the foot structure of Eosauravus 

 copei and Ichthycanthus platypus, yet there are very great differences in the phalan- 

 geal formula and the arrangement of the tarsal elements. These differences are clear 

 and indicate a separation of the two species into distinct classes. The phalangeal 

 formula in the Eosauravus, 2-3-4-5-4, is typically reptilian; while in the Ichthy- 

 canthus, 2-2-3-3-3, it is amphibian. The tarsus of the Ichthycanthus is amphibian 

 in the presence of an intermedium, but this is very small and the remaining tarsal 

 structures have -nothing which might not be found in an early reptile. There may 

 be a single or even two centralia in the reptilian tarsus among the early forms. 



The amphibian nature of the species having thus been established, it remains to 

 give a detailed account of its skeletal anatomy, with comparative references to such 

 other ancient forms as are available. Little can be said of the vertebral column, 

 since only the molds of a few vertebrae remain, and these are so obscured by a 

 closely adherent pellicle of carbonaceous material that their form can not be dis- 

 tinctly discerned. They are high, with relatively broad neural spines. There are no 

 ribs preserved. The pelvis is obscured, but it is possible to determine the presence 

 of an elongate ilium and an ischium. The leg of the left side is the best preserved 

 of all the elements, and it is to this that our attention will be confined. The oppo- 

 site leg is not so complete, yet all the long bones and a part of the tarsus are pre- 

 served with sufficient clearness to corroborate the findings of the left side. 



The femur, as has been stated, is reptilian in appearance. This is due to the 

 well-rounded articular surfaces, as though the endochondrium were well developed, 

 and to the large development of the greater and lesser trochanters, which are quite 

 prominent, though these are distorted and depressed in fossilization. The bone is 

 stout and well built and its form suggests an active habit of life. The tibia and 

 fibula are separate, and do not otherwise have sufficiently noteworthy characters 

 to call for a special description in this place, except to note an unusual anterior 

 crest on the tibia. To the lower ends of these bones articulate the first row of tarsal 

 elements, the tibiale, intermedium, and fibulare. The tarsus is composed of 9 

 elements arranged in 3 rows. The proximal row is composed of the tibiale, the 

 intermedium, and the fibulare. On the edge of the tibiale there lies a portion of 

 one of the caudal vertebras, so that the form of this tarsal element is slightly ob- 

 scured. The intermedium is a small, rounded element lying between the larger 

 elements. The fibulare is rectangular and projects a considerable distance out from 

 the tibia, but articulates directly with the large lateral distal tarsal. The centrale 

 is triangular in form and is opposed directly by the tibiale and tarsalia I to 3. 

 The phalanges are robust in appearance. The entire foot gives one the impression 

 of a very broad structure. The ungual phalanges were apparently bluntly clawed. 



