PERMO-CARBONIFEROUS VERTEBRATES FROM NEAV MEXICO. 57 



not occur, since the side shown in the present figure corresponds quite to the ventral side 

 as detennined since in various other forms. The foot Hes nearly in the position as regards 

 the leg that is shown in the present figure; possibly in life it was angulated more to the 

 tibial side, leaving more room for the fibula, which is unknown. The arrangement of the 

 bones of the tarsus agrees well with the observations of Baur on Archegosaurus, who, how- 

 ever, remained in doubt as to the presence of a fourth centrale. 



Concerning the homologies of the tarsal bones in the reptiles, there has been not a little 

 dispute, and the subject is by no means yet at rest. Gegenbaur, in 1864, considered the 

 astragalus of mammals as a composite formed by the fusion of the tibiale and the inter- 

 medium, and this view is the one usually accepted at the present time. Baur, however, 

 (Morphologisches Jahrbuch, xi, 468), after a careful examination of the embryos of numer- 

 ous reptiles and mammals, reached the conclusion that the astragalus is a single element, 

 with no evidence of fusion; that, in the Prototheria and Eutheria at least, "ein Intermedium 

 tarsi niemals embryologisch nachweisbar ist." Among certain marsupials only, and as 

 an occasional anomaly in the human foot, a small bone is found wedged in between the 

 astragalus, tibia, and fibula at the back part of the ankle joint, which Bardeleben believed 

 to be the intermedium, but which Baxu- was disposed to consider a neomorph or sesamoid. 

 In the belief that the astragalus represents but a single bone, which he was inclined to 

 consider the intermedium, Baur recognized in the "tibial sesamoid" the real tibiale. This 

 "tibial sesamoid" is a bone not infrequently found in mammals, but unknown in reptiles, 

 on the tibial side of the tarsus, articulating with astragalus, navicular and internal cunei- 

 fomi. Baur gave four hypotheses: First, that the tibiale is represented by the tibial 

 sesamoid, the intennedium by the marsupial sesamoid of Bardeleben; second, that the 

 tibial sesamoid is the tibiale, and the astragalus wholly the intermedium; third, that the 

 tibial sesamoid is the first tarsale, the marsupial bone the intermedium, and the astragalus 

 is the tibiale; fourth, that the tibial sesamoid and the marsupial bones are real sesamoids, 

 and the astragalus is the combined tibiale and intermedium. Although Baur recognized 

 the possibility of four free centralia in the amphibian foot, he did not recognize the possi- 

 bility of more than one in the reptilian tarsus, since none is known in any recent reptile, 

 nor more than one in any reptile hitherto. It seems to me that the presence of two free 

 bones between the astragalus and the inner tarsaha in the present genus offers an explana- 

 tion of the "tibial sesamoid," if it be necessary to derive the bone from a true tarsal element. 

 The first of these centralia occupies the position of the sesamoid bone in the mammals, 

 nearly, and it is not at all impossible that its j)resence may have been continuous in that 

 ]3hylum from which the mammals arose. 



In a previous paper (Amer. Permian Vertebrates, p. 44) I have expressed the opinion 

 that a free intermedium tarsi is never present in adult reptiles as a distinct bone, as based 

 upon the fact that in all the earliest-known reptiles, from the Middle Pennsylvanian to 

 the Trias, there are but two bones in the proximal row of the tarsus, bones corresponding 

 quite to the astragalus and calcaneum. Nor can I find in any later reptile, either extinct 

 or living, any certain evidence of its presence. We may safely say that if the intennedium 

 once had disappeared in the ancestral forms it never reappeared as a nonnal functional 

 element in their descendants. Any other interpretation of the known facts would require 

 the existence of two distinct phyla of vertebrates, beginning nearly as early as the Missis- 

 sippian, and continuing without break to the present time one in which the intermedium 

 had disappeared, the other in which it was persistent, but of which we have yet found no 

 certain representatives. 



Schmalhausen (Anat. Anzeiger, 1908, p. 378), from the study of pig embryos, reached 

 the conclusion: "dassbei den Vorfahren der Saugethiere mindestens eine Dreizahl der 

 Centralia Tarsi vorhanden war; das eine proximale ist jetzt im Astragalus mit dem Inter- 

 medium zusammen eingeschlossen, das zweite ist im Naviculare erhalten, und das dritte 

 kann, wie bei Schweine, mit dem Cuboid verschmelzen." The tibiale, according to him, 

 has disappeared, or is represented by the vestigial sesamoid. On the other hand. Broom 

 declares that the astragalus is composed exclusively of the tibiale, because he finds in 

 Oudenodon a small bone occupying the position of a vestigial intermedium, possibly corre- 

 sponding to the bone found in the marsupials. 



