J. Playfair MceMurrich 187 
mally, or a shifting of the insertion of antibrachial muscles distally, or, 
perhaps, a combination of both these processes. My results show that 
such a way of regarding the long flexors is erroneous and that if we are 
to obtain correct homologies we must compare only the antibrachial 
portions of the mammalian flexors with antibrachial muscles of the 
amphibia and reptilia, the palmar portions being comparable to palmar 
structures, tendons or muscles. My reason for this conclusion will be 
given in a subsequent section of this paper, but in what follows here 
attention will be directed solely to the strictly antibrachial portions of 
the mammalian flexors. 
I regret greatly that I have not been able to include a monotreme in 
the material studied, for, to judge from the descriptions to which I have 
access, they present most interesting resemblances to the conditions 
obtaining in the reptilia. The tendency toward an indistinctness in 
the separation of the superficial and deep layers of the forearm flexors 
seen in the reptilia is apparently carried further in the monotremes, 
there being recognizable in them, as distinct muscles, only a flexor carpi 
radialis, a pronator radii teres, a flexor communis digitorum, an epitro- 
chleo-anconeus and a flexor carpi ulnaris, the last in Echidna being 
united with the flexor communis to about the middle of the forearm. 
Dissections have failed so far to reveal any division of the flexor com- 
munis into constituent elements such as may be recognized in other 
mammals, and it would be interesting to determine whether or not such 
a division could be recognized in sections. Lacking information on 
this important point I must perforce take, as my starting point for a 
consideration of the mammalian muscles, a condition in which a differ- 
entiation of the flexor communis has occurred, a condition a little in 
advance of what is found in the monotremes and yet a little below 
what is found in such a mammal as the opossum, in that it fails to show 
any differentiation of the antibrachial portion of the flexor sublimis. 
I take such a condition for comparison with the lower forms rather than 
one in which the forearm portion of the sublimis is differentiated, 
because this muscle is peculiar to the mammalian series and possesses 
within that series a somewhat complicated development which may 
more conveniently be considered later on. 
The arrangement of the muscles in the somewhat hypothetical condi- 
tion may be supposed to be as follows. Superficially upon the ulnar 
side of the forearm is the flexor carpi ulnaris (Fig. 4, F.C. U.) arising 
by two heads, one from the internal condyle of the humerus and the 
other from the olecranon process, and inserting below into the ulnar 
side of the carpus. In close proximity to this muscle is the epitrochleo- 
