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Show 468 MR. G. W. BUTLER ON THE SUBDIVISION OF [Nov. 19, the case of Birds and Mammals), I give for what it may be worth the following interpretation, based mainly on the study of very young specimens either recently hatched or still within the egg. I have examined but one well-preserved adult specimen (of the Alligator type), and some eight or more small animals, four of which were unhatched, three of them not having yet cut their teeth, and being possessed of the horny egg-breaker on the snout. Young animals such as these are, I think, best for making out the relations, as not only is it easy to cut longitudiual and transverse sections of them, but in the adult the true relations of the membranes tend to become obscured by adhesions or other adaptive changes, and tbe only drawback is the caution necessitated by the delicacy of their membranes. That which first strikes the observer with regard to the body-cavity of a Crocodile, is the subdivision of the Pleuro-peritoneal cavity into Pulmohepatic and Intestinal portions by a post-hepatic septum l; secondly, the facts mentioned by Huxley (4, p. 568), that the gizzard is firmly connected with the body-wall, so that it appears to be itself shut off from the intestinal cavity; and that the liver projects into a number of different sacs2. In these three points the Crocodiles at first sight approach the Birds rather than other Sauropsida. It appears to me, however, that the Crocodiles, in the matter of the subdivision of their body-cavity, are distinctly reptilian rather than avian ; and that the only satisfactory way of comparing the two types, in the absence of the much needed embryological data (cf. suprel, p. 453), is to analyze the complex condition of each into its component elements, and to compare these in the light of our knowledge of the simpler Sauropsida (Lacertilia), and of the development of the bird. Fig. 35 represents one of the young Crocodiles referred to reduced one half, the lines indicating the approximate planes of the sections sketched in the corresponding figures, which are on a scale three times as large3. Fig. 42 is intended to show the cut edges of the pleuro-peritoneal membrane, as they would appear on the removal of the ventral body- 1 Hunter in 'Essays and Observations on Natural History ' (edited by Owen), vol. ii. pp. 386 & 337, gives a careful account of the relations and attachments of the liver in the adult Crocodile. H e emphasizes the fact that the liver is shut off from the abdominal cavity, and says that it itself makes a kind of diaphragm. He adds, however, that on account of the well-marked character of the membranous lamella behind it, we m a y "consider the liver as in the thorax." 2 Cf. Owen, P. Z. S. 1831, pp. 137 and 169. I was not aware, until after the present paper was in type, that this author had described some of the anatomical features herein discussed. 3 The longitudinal sections in question, figs. 39, 40, & 41, are drawn from the ventral side, as are also figs. 42 & 43, and also fig. 29 of the duck, and 31 of Tupinambis. All these differ from the horizontal sections of the developing bird, which are drawn from the dorsal side. This difference being once noted, I hope that there will be no difficulty in making any comparisons that may be desired. The transverse sections of the Crocodile, figs. 36, 37, & 38, agree with those of the developing bird, and also with figs. 45 & 47 of the adult, and 32 of Tupinambis, in being drawn from behind. |