THE COAL MEASURES AMPHIBIA OF NORTH AMERICA. 



h 



(q) The leg (fig. 21, B) is composed of the femur, tibia, fibula, and 5 digits. The 

 tarsus is usually cartilaginous, a single osseous tarsus (483, 484) being known 

 (plate 23, fig. i) from America. The distal phalanges may or may not be clawed, 

 depending on the habits of life of the animal. The elements of the leg are ossified 

 in a similar manner to those of the arm. 



(r) The ventral scuteUation (fig. 9), so commonly present among all groups of 

 Amphibia in the Coal Measures, consists of a series of ossifications or calcifications 

 in the myocommata. Among modern amphibians they occur as thin perpendicular 

 planes of connective tissue which are sometimes cartilaginous, especially in Necturus, 

 regarded by Wilder (Memoirs of the Boston Society of 

 Natural History, vol. v, No. 9, p. 400, fig. 6, 1903) and 

 by Wiedersheim (605, p. 58) as a homologue or predecessor 

 of the sternum, although Wiedersheim says: 



"The sternum appears for the first time in Amphibians in 

 the form of a small variously shaped plate of cartilage situated 

 in the middle line of the chest. It arises as a paired cartilaginous 

 plate in the inscriptiones tendineas of the rectus abdominis 

 muscle, and therefore may be looked upon as corresponding 

 to a pair of 'abdominal ribs.' Such cartilaginous abdominal 

 ribs must have been present in greater numbers in the ancestors 

 of existing Urodeles." 



This supposition is fully sustained by the anatomy of 

 the Branchiosauria (459), which must be looked upon as 

 the actual ancestors of the Caudata. Wilder says of these 

 structures in Necturus (op. cit., p. 400): 



"The several cartilaginous rudiments which represent this part (i.e., sternum) in Nec- 

 turus are somewhat difficult of detection and thus entirely escaped the attention of the 

 earlier investigators. They consist of a number of thin cartilages found in several suc- 

 cessive myocommata of the pectoral region and confined mainly to the area covered by the 

 overlapping epicoracoids." 



The homologue of the ventral scutellce is found in plesiosaurs, crocodiles, Sphcnodon, 

 and other reptiles in the "abdominal ribs," and the same myocommatous ossifica- 

 tions undoubtedly go to the formation of the chelonian plastron. What the causes 

 were which produced the development of the ventral scutellae to such a high degree 

 among the primitive land vertebrates is uncertain, but they are certainly more 

 highly developed among the primitive reptiles and amphibians than among the 

 later members of those classes. Among the Amphibia of the Coal Measures they 

 attained, in some forms, a high degree of development and differentiation. They 

 are present in all families so far known, except the Tuditanidae, in which the myo- 

 commata may have been cartilaginous. The Sauropleuridas present the highest 

 development of these structures among the American forms, in which the scutes are 

 large and osseous. Among the Branchiosauria they are calcified or partially ossi- 

 fied and are always arranged en chevron on the belly, chest, arms, and throat, their 

 arrangement and direction of the chevron being modified according to the myomeres 

 of the various regions. The ventral scutellae of the European Branchiosauria are 

 figured and described fully by Credner (192, p. 21, figs. 4 to 1 1). 



FIG. 9. Ventral scutdlae of Micrcr- 

 pclan caiidatum, a Coal Meas- 

 ures salamander from Mazon 

 Creek. X 5. /, femur; h, hu- 

 merus; Is, lines of scutes; v, 

 vertebral column. 



