GREGORY, PRESEXT STATUS OF ORIGIX OF TETRAPODA 36? 



cephali, and their allies. The salamander Xecturus, probably the nearest ap- 

 proach to this series represented by living fauna, offers in its distal muscles 

 some few suggestions of an earlier phylogenetic stage, and is thus of funda- 

 mental importance in the present inquiry. The well-nigh complete correspond- 

 ence in the fore and hind limb as regards not only bones and muscles, but 

 other parts as well, has been commented on above and offers strong support 

 for the doctrine of serial homology, to be considered later. There are, also, as 

 is the case with higher forms, some traces of a correspondence between the 

 dorsal and ventral surfaces of a single paw, giving a suggestion of the deri^•a- 

 tion of the chiridial musculature from a fin-like precursor in which the jointed 

 rays (digits) were supplied by similar muscular elements applied both dor- 

 sally and ventrally, as in present-day fishes. The following description is that 

 of the anterior limb, but with the substitution of the terms tihia and fibula 

 for radius and ulna, tarsus for carpus, and so on, it will be found almost 

 equally applicable to the posterior one. In a few cases a muscle which is we)l 

 developed in the anterior limb is small or wanting in the posterior, and thus 

 the former is a little more typical.^ 



"Reviewing the conditions in this, probably the most primitive chiroptery- 

 gium now left to us, several interesting points become manifest. The digits 

 are moved in two ways, either flexed and extended or moved sideways, but 

 while the system which provides for this latter form of motion is extremely 

 well perfected, that for flexion and extension is not. For abduction and ad- 

 duction there are topically five separate muscles for each digit, that is, two 

 ventral, two dorsal and one intermetacarpal, while for flexion and extension, 

 aside from the system supplied by an aponeurosis, and evidently a newly in- 

 troduced feature, there are but three. This extreme perfection of the sideivays 

 movement of the digits in the most primitive chiridium known, together ivith 

 the iveak and makeshift arrangements for hending and straightening the digits, 

 strongly suggest the derivation of the chiridial type from one in which the 

 digits ( fin-ray sf) required to 'be constantly opened and shut by lateral move- 

 ments, precisely as in the case of the fins of most fishes. 



''During later phylogenetic history there is an evident tendency to increase 

 the efficiency of the fiexor-extensor system and diminish that of the abductors 

 and adductors, except in the case of the two digits that form the ends of the 

 series (I and V), and the most of these changes have already occurred among 

 the higher urodeles." 



As the skeletal remains of the limbs of Carboniferous Tetrapoda retain 

 but little that is clearly suggestive of derivation from the paired appen- 

 dages of fish, so too the footprints of these animals indicate that the 



8 "In one point the free limb of Nectunis diverges from what is generally believed to 

 be the typical cbiropterygium, and that is. it possesses but four digits in each extremity 

 instead of the canonical five which is usually considered primitive. Since the nearest 

 ally of this species, the cave form, Proteus, exhibits a still gieater reduction of digits 

 (anterior, 3; posterior, 2), it has been presumed that this is in both cases a secondary 

 reduction. Certain facts, however, lead one to think that the first land vertebrates pos- 

 sessed a smaller number of digits than five, and if this be so, the condition in these two 

 salamanders is primitive, and not a secondary reduction. According to the reduction 

 theory digit I is assumed to be the one lost, and in accordance with this the four digits 

 present are designated here, both in text and illustrations, as II-V." (Wilder, p. 231). 



