Page:Proceedings of the Royal Society of London Vol 69.djvu/19

In A there is only one foramen, and through it pass the ventral tranches of the nervus collector. In type B there are two foramina. Through the more lateral one pass the ventral branches of the collector, whilst the more median and posterior foramen gives passage to the nerve immediately succeeding the collector. This arrangement is the most usual one, and corresponds to that figured by Davidoff. In type C there is but one foramen, through which passes the ventral portion of the nerve succeeding the collector. The ventral division of the collector itself passes now through a more or less pronounced notch on the lateral border of the girdle.

Now according to the views of those who uphold the " side-fold " theory of limb origin, and explain the varying position of the girdle by assuming inter- or ex-calation of vertebrae, the pelvic girdle has been derived from the hypothetical segmental blocks of cartilage which were formed in the side-fold. As some of these coalesced and grew in medially to form the girdle they came to enclose the nerves which originally ran down between them. Consequently we may look to the nerves piercing the girdle as some guide to the number of morpho- logical units which, on this theory, go to make up the girdle.

According to this view the girdle in Acanthiax is composed of at least three such units (fig. 4, x, y, z). And to fit the facts it must be assumed that either v or z is capable of being dropped out. Conse- quently this theory involves the necessity of supposing that the pelvic