192 THE CLASSIFICATION OF THE OPHIDIA. 



to the right of the heart, so that in this genus also it may represent the rudimental 

 lung of the Colubroidea. There is here no tracheal lung or organ. 



The rudimental lung is often concealed from view and difficult to discover. The 

 best test of its presence is the foramen which connects it with the trachea, which will 

 generally be found piercing the cartilage of the latter near the apex of the heart. The 

 rudimental organ may then be found by inserting a bristle and observing its destina- 

 tion through the more or less transparent tissues. In but one instance have I found a 

 rudimental lung without a connecting foramen, viz., in the Mexican Ficimia olivacea. 

 On the other hand, the foramen may terminate in a small blind sac. 



The pulmonary characters may be determined without much dissection. The 

 position of the heart must be first ascertained and a longitudinal median incision made 

 in the abdominal wall. In all forms except the Epanodonta and Catodonta the trachea 

 will be found passing to the left side of the heart and entering the lung near its apex. 

 By splitting the trachea, not too near its abdominal border, on turning the free margin 

 upwards as the snake lies on its back, the foramen bronchial? will be seen and its lumen 

 can be explored. The trachea is concealed by the oesophagus, which must be drawn 

 to the left side of the body in order to make the examination. The examination of 

 the tracheal lung requires the division of the abdominal wall further towards the head 



The tracheal lung greatly extends the surface available for blood aeration. This 

 is useful to snakes for the reason that the huge masses of food which they ingest 

 so compress the true lung that another organ is necessary. Most snakes, whether 

 they have a tracheal lung or not, have the pulmonary organ greatly elongated, so that 

 while one portion is compressed by the contents of the alimentary canal another part 

 is free to function. The tracheal lung enables the snake to inflate the anterior 

 part of the body. This is conspicuous in the true venomous species (Solenoglypha). 

 In the same way Heterodon inflates its huge diverticulum. In the marine water 

 snakes Acrochordus and the Hydrophidse these organs serve as floats. In the fresh- 

 water snakes (ISTatricinse) there is no tracheal lung. 



III. HISTORY AND ACKNOWLEDGMENTS. 



The first paper which called attention to the importance of the penial characters 

 as indications of affinity in the Ophidia was published by me in the American Natur- 

 alist for December, 1893. The relations of the pulmonary structures to the systematic 

 relations of the Ophidia were first pointed out by me in a paper published in the Pro- 

 ceedings of the American Philosophical Society for June, 1891. In the American 

 Naturalist for October, 1894, I published an amended classification of the two sub- 

 families, Xenodontinse and Philodryadinae. In the Proceedings of the Philadelphia 



