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Show 1889.] PR/EPOLLEX AND PRHEHALLUX. 261 studied these matters, and has compared the different degrees of reduction in the mammalian hand and foot, will admit that not only the reduction from five to four, three, two, and even one digit is possible, but equally the reduction from seven to six, and from six to five. I have also found in many Reptiles the rudiment of a praepollex and a praehallux, although most of these animals are much differentiated. W e shall probably never find either in Mammals or in Reptiles (Lchthyosaurus excepted) seven equally developed digits ; but nobody will deny that a great many forms are lost for ever, and that we have little chance of finding complete fossil remains of digits in animals whose hands and feet were wholly or partly cartilaginous, as is the case with most Amphibia. I have also found the praepollex in an animal the position of which among the Vertebrata is very doubtful; I refer to the Meso-zoic Theriodesmus phylarchus of Seeley (Phil. Trans, vol. 179, B, 1888, p. 141). There is preserved in the British Museum a natural mould of the bones of the right forearm and hand of this animal, together with other fragments of its skeleton. Prof. Seeley's description appears to me to be inexact, and I cannot agree either with his views or with his restoration of the carpus. He considers that there are three centralia. His first centrale is, as my reconstruction shows (cf. Plate X X X . figs. 4 & 5), the lunar; his third centrale, on the border of the scaphoid (" scapho-lunar " of Seeley) and the trapezium, is the proximal bone of the praepollex ; and his second centrale appears to m e to consist not of one bone, but of two, i. e. two centralia. The first1 centrale has the same position as in all Mammals in which it is a distinct bone; he second is placed as in the carpus of Centetes (in which it is not quite distinct),-and as in the tarsus of Cryptoprocta ferox (Madagascar; Leyden Museum), in which I have found a distinct bone (triangulare tarsi). A small ossicle (small, perhaps, only in the plane in which the stone has been cleft) lies on the border of the trapezium ; it has been omitted by Prof. Seeley in his diagrams (I. c. pp. 147 and 150), and is in reality the distal bone of the praepollex (Pp.d.). Figure 4, which I have had redrawn from the original specimen, also shows some other interesting points. The unciform (u.) bears a condyle for articulation of the fourth, and a fossa for that of the fifth metacarpal bone. Between the first and second, and also the second and third phalanges of the third finger, and between the first and second phalanx of the fourth finger (the end of which cannot be clearly made out), there are intermediate pieces of bone which are probably epiphyses. In the other fingers these intermediate bones are coalesced with the phalanges in the same manner as the epiphyses of other Mammals. If these intermediate bones are epiphyses, the phalanges of the third and fourth fingers of this animal would appear to bear epiphyses at both ends-a condition rarely seen among Mammals. 1 I enumerate the centralia from the radial or tibial border, as is customarily done in dealing with the digits and the metacarpal and metatarsal bones. PROC. ZOOL. Soc-1889, No. XVIII. 18 |
