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Show 1904.] ANATOMY OF CERTAIN SNAKES. I l l two arrangements of the intercostals, the Boine or the Pythonine, is the more primitive ; perhaps it is the former. In any case it is from the Boine rather than the Pythonine type that the intercostal system of many Colubrine Serpents is derivable. Eryx conicus differs in detail from Eryx jaculus in the arrangement of the intercostal arteries. Anteriorly, the arteries are given off singly and regularly, bifurcating just before their entry into the thickness of the parietes. There is with great regularity one to each vertebra. The only exception that I noticed was in the case of one intercostal arising from the right aorta in front of the junction of the two aortae, and of another some way behind ; these supplied two intervertebral spaces. This apparently is the only trace left anteriorly of the arrangement characterising Eryx jaculus. Far back, much further than in E. jaculus, the paired arrangement of the intercostals is seen. Still it is evident that the two species do not differ in the type of the arrangement of the intercostals. Eryx johni again shows a fundamental likeness to, but detailed differences from, the other two species of the genus. The general agreement is that the anterior series of intercostals arise singly from the aorta and bifurcate only just before entering the parietes. They begin to be double in origin shortly behind the liver. In the region where they are double they are frequently asymmetrical in both size and in point of origin from the aorta. Thirteen intercostals arise from the right aortic arch before its union with the left. There is no trace of any formation of an azygos median vertebral artery such as occurs in Eryx jaculus. The intercostal system of this species in fact is somewhat intermediate between those of the two other species of the genus. § Some Visceral Arteries. (Esophageal arteries.-I have examined these arteries carefully in an injected example of Eryx jaculus. It agrees with other snakes and with the Lacertilia in the fact that the intercostals (already described on p. 109) arise from the right aortic arch only. On the other hand, branches to the oesophagus, which are represented in the drawing (text-fig. 19, p. 109), arise either directly or indirectly from both arches. I have not observed this double origin of the oesophageal branches in any other Ophidian ; but, as I am not quite in a position to deny its occurrence, I cannot emphasise the fact as a characteristic of the Boidse. It is, however, 1 am inclined to think, an anatomical feature not found in the Lacertilia, The oesophageal vessels, or rather vessel (for I only noticed one), arises from the left aorta ; it passes back along the oesophagus, giving off branches to that gut, and becomes continuous with the first of the oesophageal arteries arising from the conjoined aortse. The right aorta does not directly give off oesophageal arteries. But from two of the intercostals of the right aortic arch such arteries arise. |