42 MR. F. E. BEDDARD ON THE [Jan. 16, 



facts which necessitate a reconsideration of this view, and which 

 tend to destroy its apparent obviousness. 



Cope*, to whom our knowledge of the headwards extension of 

 the lung is mainly due, though the fact of this extension in the 

 Yiper was known fifty or sixty years earher, terms this section of 

 the lung the " tracheal lung," and after a survey of the leading- 

 groups of Ophidians found it to occur in the principal subdivisions 

 of the order or suborder. He found this tracheal lung in Ungalia 

 among the Boidjse, " in the Solenoglypha without exception," and 

 in several Colubrines, to which I myself have added the 

 Hamadryad f. The occurrence of these tracheal lungs so widely 

 among the Ophidia suggests a retention of a character rather 

 than its independent development in the several groups. So fai-, 

 however, one can do no more than incline to the former view. 

 There are, however, other facts. In the first place, among Snakes 

 generally the rings of the trachea, where there is no tracheal lung, 

 are incomplete posteriorly, leaving a gap filled in with soft tissue. 

 This soft tissue is continuous with the lung-tissue where the latter 

 commences, in these cases near to the heai't. 



It might be held — if the matter ended here — that the non- 

 junction of the tracheal rings posteriorly had no more significance 

 than the failure to join posteriorly of the tracheal rings in the 

 Cassowary % or in Man §. But a few cases seem to show that this 

 failure to join is of meaning as the last term in a series. For in. 

 some Serpents, e. g. in Lioheterodon, there is not merely a failure 

 to unite posteriorly among the tracheal rings, but the membranous 

 space left is of wide dimensions, much wider than the actual 

 trachea, and fully as wide as the tracheal lung where that organ 

 is developed. Moreover, in this snake there are traces of a develop- 

 ment of diverticula of the cavity such as are to be met with in a 

 much more fully developed condition in the Hamadryad snake i|. 

 These facts therefore afibrd some evidence that the tracheal lung 

 was formerly more widely spread among the Ophidia than it is now ; 

 that it is not a new structure in those forms where it occui'S, but 

 an archaic structure so far, at any rate, as Snakes are concerned. 



It will be observed, moreover, that there is a distinct relation 

 between the development of the neck pai't of the lung and the 

 asymmetry of the lungs. This relationship, however, does not 

 after all amount to a great deal ; for the only Serpents in which 

 there are a pair of well- developed thoracic lungs are the Boidse. 

 It is nevertheless noteworthy that among these primitive Snakes, 

 as they are held to be, the genus Ungalia, which possesses the 

 tracheal lung, is, like the Colubrine Snakes, without moi-e than 

 a rudiment of one of the lungs. The only allied form in which 

 this asymmetry of the lungs is known to exist is Ilysia ; but in 

 Hysia there is no development of the tracheal lung. My object, 



* Proc. Amer. Phil. Soc. 1894, p. 217. 



t P. Z. S. 1903, vol. ii. p. 319. I Forbes, P. Z. S. 1881, p. 783. 



§ Treatises on Human Anatomj'. 



[| Beddard, P. Z. S. 1903, vol. ii. p. 319. 



