474. The Phylogeny of the Palmar Musculature 
into the metacarpo-phalangeal cartilage of the same digit. ‘There is a 
possibility that the muscle to the pollex which I have referred to the 
stratum profundum of the middle flexor may really belong to the flexor 
profundus series, but its general relations seem rather with the medius. 
The index possesses two muscles of the profundus set. One of these 
(Figs. 3 and 5, fbp3) arises from the radial side of the base of the 
second metacarpal and inserts into the ulnar side of the metacarpo- 
phalangeal cartilage of the first digit. In Callisaurus this muscle con- 
sists of two fairly distinct portions, one of which has an origin both 
more distal and more upon the volar surface of the metacarpal than 
the other and probably represents the middle slip of the amphibian 
muscle. The second muscle (fbp2) is more slender than its fellow and 
arises from the ulnar side of the metacarpal and inserts into the ulnar 
side of the metacarpo-phalangeal cartilage of the second digit. 
In the third and fourth digits the arrangement is similar; the muscles 
which arise from the radial side of the corresponding metacarpals 
(fbp; and fbp;) apparently include a median slip as well as a radial 
and extend to the ulnar side of the second and third digits respectively, 
while two other muscles (fbp3 and fbp,,) arismg from the ulnar and 
volar surfaces of the metacarpals pass to the ulnar side of the proximal 
phalanx of their own digits. 
In the fifth digit the muscle which passes across to the fourth digit 
(fbp;) is relatively strong and is readily recognizable, but that which 
passes directly to the proximal phalanx (Fig. 5, fp) is not so easily 
distinguishable, being more or less concealed by and associated with 
the part of the flexor brevis superficialis which constitutes the abductor 
minimi digiti. In Iguana and Callisaurus the profundus brevis muscle 
was quite distinct from the abductor though completely covered by it, 
in Iguana, indeed, almost enclosed by it (Fig 5). In Liolepisma, how- 
ever, I was not able to separate the two muscles, which present relations 
recalling those found in Amblystoma. One would naturally be inclined 
to regard the amphibian arrangement as the more primitive one and 
that found in Callisaurus and Iguana as derived by a separation of an 
originally simple muscle into two portions, but although I have not 
been able to exclude this possibility yet I am more inclined to believe 
that the distinctness of the two muscles is really the more primitive 
and that the arrangement which occurs in Amblystoma and Liolepisma 
is the derived one. 
My reasons for this view are based partly upon the relationship which 
the portion of the amphibian muscle bears to the flexor carpi ulnaris. 
This muscle belongs to the superficial layer of the forearm musculature, 
