Page:The Osteology of the Reptiles.pdf/188

170 seems to indicate the original presence of another bone that would correspond better in position with the real pisiform. Unfortunately, the single known specimen of the carpus of Trematops has but two bones preserved in the proximal row. Two centralia, and two only, were present in Trematops, while the specimen of Eryops, here figured by the kindness of Dr. Gregory, has a small space which may represent a small third centrale. There are five well-developed carpalia in both forms, proving conclusively the presence of five digits in the hand.

In some of the early cotylosaurs and anomodonts, supposed water habits have delayed the ossification of some of the mesopodial bones—supposedly, but it is a curious fact that all have similar feet, short, broad toes and ungual phalanges, very broad humeri, and short epipodials. It is not impossible, it seems to the author, that similar walking or digging habits, more after the mode of the tortoises, had, even this early, brought about a modified structure in the carpus and tarsus of Limnoscelis (Fig. 133), Diadectes, and Lystrosaurus.

It is the general belief that the loss of mesopodial bones has been due to their fusion with adjacent ones; it is doubtless true for the most part, but not always. That there has been an actual loss of the first centrale and fifth distale, the first to be lost in both carpus and tarsus, seems certain, as shown in specimens of various early reptiles, where unoccupied spaces for cartilaginous elements have been preserved. Moreover, their actual loss in living reptiles has long since been affirmed. Primitively both centralia were large (Figs. 133, 134, 137 ), as was also the fourth carpale, as correlated with the longest and strongest digit. And this carpale is the most persistent bone in the carpus, as it also is the first to be ossified in the human wrist. Every other bone of the carpus may be absent in different reptiles, but not the fourth carpale, unless it be in certain quadrupedal dinosaurs like the Sauropoda (Fig. 141 ) and Stegosauria (Fig. 141 ).

There was a perforating foramen between the second centrale, intermedium, and ulnare that was very persistent, retarding or preventing the fusion of these three bones (Fig. 134).

The first centrale and fifth carpale are always absent in mammals, at least since early Eocene times, but the second centrale is often