1909.] OP SNAKES OF THE GENUS BOA. 925 



in this region of the body, though not in any way comparable to 

 the secondary longitudinal trunks already described in the more 

 anterior region of the body. Laterally, and here and there, there 

 ai*e slender longitudinal arteries connecting a successive series of 

 intercostals at some distance fi'om their origin from the aorta. 

 This, it will be seen, is quite a different thing to the conditions 

 described in the more anterior region of the body where the longi- 

 tudinal trunks occur before the intei'costals are given off. This, 

 then, is the state of affairs which the intercostal arteries of Boa 

 occidentalls show. In the Madagascar species the intercostals are 

 different in their origin anteriorly. In the liver region of the 

 body the intercostals arise regularly from the aorta itself, and 

 vary as to whether they arise actually in pairs or from a single 

 branch which shortly divides in a Y-like fashion to form the 

 intercostal of each side. This is shown in the accompanying 

 figure (text-fig. 284), which represents a piece of the aorta in the 

 anterior region of the body. In the kidney region the inter- 

 costals are also given off in regidar pairs from the aorta itself, 

 a,nd are paired from the very first. Tlius Boa madagascariensis 

 agrees with B. occidentalis in the origin of the more posterioily 

 situated set of intercostals, but differs in the origin of the more 

 anteriorly situated set. We find, therefore, that iu the inter- 

 costal arteries, as in the structure of the lungs, the Madagascar 

 species of Boa differs from the Neotropical Boa occidentalis. It 

 is furthermore important to note that the Neotropical species of 

 Boa described in the present paper agrees in the points just 

 raised with the other two JSTeoti'opical species, viz., B. diviniloqua 

 and B. constrictor. A difference in geographical position thus 

 corresponds with certain definite structural differences. Finally, 

 it is not without interest to observe that there is a very close 

 parallel between the New World and Old World Boas on the one 

 hand, and the New World and Old World Corallus on the other 

 hand . 



The parallel, moreover, is more exact than that which can be 

 drawn in the case of the lung. For the difierence in the inter- 

 costal vessels between the Boas of the Neotropical region and 

 that of Madagascar is quite the same as that between the two 

 Neotropical species of Corallus which have been studied and their 

 Madagascar representative. I have already described the latter 

 facts in some detail*, and refer to my description. Had the 

 parallel in the structure of the lungs been as close as that which 

 the arterial system shows, it might have been peimissible to 

 consider the question whether the Madagascar Boines of both 

 genera were not better included in the same genus. At present, 

 however, this alteration of existing views seems to me to be 

 prematvire. 



* "A Comparison of the Neotropical Species of Corallus, &c.," P. Z. S. 1908, 

 p. 135. 



