1889.] PR/EPOLLEX AND PR^HALLUX. 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 prsepollex 

 and a praehallux, although most of these animals are much dif- 

 ferentiated. We shall probably never find either in Mammals or in 

 Reptiles {Ichthyosaurus excepted) seven equally develoi)cd 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 carti- 

 laginous, as is the case with most Amphibia. 



I have also found the prsepollex in an animal the position of 

 which among the Vertebrata is very doubtful ; I refer to the Mcso- 

 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 recon- 

 struction shows {cf. Plate XXX. 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 prsepollex ; and his 

 second centrale appears to me to consist not of one bone, but of two, 

 i. e. two centralia. The first^ 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 Ce7itetes (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 l)een omitted by 

 Prof. Seeley in his diagrams (/. c. pp. 147 and 150), and is in reality 

 the distal bone of the jirsepoUex (Fp.d.). 



Figure 4, which I have had redrawn from the original specimen, 

 also shows some other interesting points. The unciform (m.) 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 inter- 

 mediate 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. 



^ 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 



