468 MR. G. W. BUTLER ON THE SUBDIVISION OF [Nov. 19, 



the case of Birds and Matnmals), I give for what it may be worth 

 the followiug 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 the 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 ^ ; 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 sacs '. 



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 (c/". supra, 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 Saurop- 

 sida (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 large ^. 



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- 



^ Hunter in ' Essays and Observations on Natural History ' (edited by 



Owen), vol. ii. pp. 3o6 & 337, gives a careful account of the relations and 

 attachments of the liver in the adult Crocodile. He emphasizes the fact that the 

 liyer is shut off' from the abdominal cavity, and says that it itself makes a tind of 

 diaphragm. He adds, however, that on account of the well-marked character 

 of the membranous lamella behind it, we may " consider the liver as in the 

 thorax." 



- 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 anato- 

 mical features herein discussed. 



^ 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 

 Tiqnnanihis. All these diifer from the horizontal sections of the developing 

 bird, which are drawn from the dorsal side. This difference bemg 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 develojiing bird, and also with figs. 45 & 47 of the adult, and 32 of 

 TupinaTnbis, in being drawn from behind. 



