1889. ] PREPOLLEX AND PREZHALLUX. 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 przpollex 
and a prehallux, although most of these animals are much dif- 
ferentiated. We shall probably never find either in Mammals or in 
Reptiles (Ichthyosaurus 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 carti- 
laginous, as is the case with most Amphibia. 
I have also found the prepollex 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, 1883, 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 (ef. 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 preepollex ; 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 Centetes (in which it is not quite distinct),—and as in 
the tarsus of Cryptoprocta feror (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 (J. ¢. pp. 147 and 150), and is in reality 
the distal bone of the preepollex (Pp.d.). 
Figure 4, which I have had redrawn from the original specimen, 
also shows some other interesting points. The unciform (w.) 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. 
1 T 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. Zoou. Soc.—1889, No. XVIII. 18 
