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Show 374 PROF. K. VON BARDELEBEN ON THE [Apr. 17, Ph., and in Pedetes there is a true nail (in some specimens only a nail-like structure). The resemblance of Pp. and Ph. to a reduced thumb or great toe is very striking (e.g. foot of Carnivora). If the first digit of the mammalian hand and foot had always been lost as a true digit, and if w e knew only mammals with four digits, then w e might be in the same doubt about this reduced structure-"prae-index"-as many of m y colleagues are n o w about m y praepollex. The Pp. and Ph. are much better developed in lower mammals than in higher ones-they are present and free in primitive types ; they are lost or become united with their neighbours in higher or more differentiated mammals, or they get the appearance of " sesamoid bones." In lower mammals the Pp. may consist of two bones, in higher there is always only one bone. The Pp. consists of at least two bones in Theriodesmus phyl-archus, the position of that animal being not yet sufficiently ascertained. Five years ago (P. Z. S. 1889) I supposed it to be a Promammal, but, as Prof. Seeley kindly told m e this March, there is now evidence for this interesting animal being a true reptile. W e must make a clear distinction between the flssion of digits which occurs in Cetacea (and Ichthyosauria perhaps) and the existence of rudiments of digits. In Cetacea there is also a real Pp., as Prof. Kukenthal has shown. It may be that the Pp. and Ph. of many animals represent not only an old (reduced) structure, but also a partly new one, and that only the basis of the Pp. of Pedetes was inherited in those species and that it gradually increased in size. There are many muscles in the mammalian hand and foot which have to do with the Pp. and Ph. exclusively or nearly so. W h e n the Pp. and Ph. disappear or coalesce with other bones, these muscles may also vanish or they may become united with other muscles, or they may get inserted in those parts of bones which have been originally free and more or less movable, as e. g. Pp. and Ph. Concerning the nerve-supply of muscles I found that the following muscles are provided with two nerves :- the flexor digitorum superflcialis brevis manus ") et pedis in the palmaris longus or flexor digitorum super- I •, ficialis longus the ulnaris internus (flexor postminimi) the flexor digitorum sublimis the flexor digitorum profundus, in aU mammals. mammals. Concerning the homology of the true carpal and tarsal bones I give here a table which, though it may be incomplete, I hope may contain definite results:- |