256 MR. F. E. BEDDARD ON THE [May 1, 



however, the muscle is attached to the sternum and its fibres are 

 striated (? Penguin) ; as to the Crocodile, Prof. Huxley does not 

 state whether the muscles are striated or not. Judging from the 

 analogy of other reptiles (Lacertilia), where muscular fibres also 

 extend into the mesenteries ', they are not ; but the homology is 

 not, in my opinion, already stated, affected by this question^. 



The rudimentary muscles which clothe a limited area of the 

 oblique septum in the Duck and Toucan are probably to be derived 

 from the same muscle in the Crocodile, or else they may be directly 

 traceable to a bird ancestor in which the same muscles were present 

 in the same hypertrophied condition that they now are in the 

 Penguin and Puffin. 



The muscle in question is in fact more highly developed in the 

 Puffin and Penguin than in the Crocodile ; but the habits of the 

 birds suggest an explanation not only of the retention of the muscle 

 but also of its great development and the appearance of an attach- 

 ment to the ventral parietes. 



Both these birds are diving birds, and it seems therefore reasonable 

 to suppose that any organ which would facilitate vigorous inspirations 

 and expirations would be highly advantageous. Now the respiratory 

 movements in birds are largely brought about by the abdominal 

 muscles, which depress the sternum and the parietes, and so drive the 

 air from the air-sacs through the lungs to the exterior. But the 

 sternum in the Puffin &c. is long and the muscular abdominal parietes 

 are therefore shorter than usual. The muscular force available is as 

 a consequence not so great as in many other birds (e. g. the Emu) ; 

 this deficiency is made up for by the muscle covering the oblique 

 septum, and moreover the relations of this muscle are such that it is 

 particularly available for its presumed function. 



In the first of the present series of " Notes upon the Visceral 

 Anatomy of Birds''^ I have called attention to the resemblance 

 between the so-called " omentum " of Birds and the " horizontal 

 septum " of the Crocodile, which is directly continuous with the 

 obhque septa of the same reptile, and is apparently not distinguished 

 from it by Prof. Huxley. It seems to me that the entire fibrous 

 expansion which arises from the vertebral column and extends over 

 the anterior face of the stomach, liver, ^c. in the Crocodile represents 

 both the oblique septa and the omentum in the bird. 



The middle part of the fibrous expansion in the Crocodile bears 

 the two anterior abdominal veins, or at least they pass between it 

 and the ventral parietes. In the bird there are a number of small 

 veins upon the omentum which join the portal system, and are 

 probably collectively the equivalents of the anterior abdominal svstem 

 in the Crocodile. This appears to me to be an argument in favour 

 of identifying the median portion of the fibrous expansion in the 

 Crocodile with the omentum of the bird. 



' Rathke, Wiener Sitzungsb. 1852. 



^ Prof. Gr. B. Howes tells me that the muscle in question is composed of 

 plain fibres in the Crocodile. 

 » P. Z. S. 188.9, p. 836. 



