1150 REPORT OP NATIONAL MUSEUM, 1898. 



wliicli are all of rather small dimensions, excej^t the first named. Two 

 others are confined to the Lower California peninsula, G. mitchellii and 

 C. enyo, while two are restricted to the Mexican plateau and its western 

 slope, the C. imlystictus and C. hasiliscus. 



The variations in structure of these species is not great, and they 

 form a compact generic division. Dr. Coues has proposed to separate 

 the C. cerastes as type of a separate genus, distinguished by the pro- 

 longation of the free border of the superciliary plate into a horn-like 

 process. Were this process distinctly articulated at the base from the 

 superciliary plate, as is the case in the viperine genus Cerastes, we 

 should be compelled to adopt such a division ; but at present I do not 

 see the way to separate it, especially as the process is often but little 

 l)ronounced. I have also proposed a generic division for the G. Jepidns 

 based on the single nasal plate, but the plate is sometimes divided in 

 part, and for the present I do not adopt the division, or at least until 

 I can see more specimens of the species. 



The origin of the curious epidermal structure at the end of tlie tail, 

 known as the rattle, is as yet a subject of speculation only. We have, 

 however, so many rudiments of it in other generic divisions oi the Cro- 

 talidic that its origin from some of these is evident, and that it occurred 

 at no very late period of geologic time is probable. The terminal cau- 

 dal vertebra^ are coossified, compressed, and expanded into a vertical 

 body, which enters the first or basal button of the rattle. Such a mod- 

 ification is found in a rudimental condition in the genus Lachesis, where 

 it is covered by a simi)le horny sheath, grooved at the sides. In 

 Sistrurns the rattle is of absolutely and relatively small size, and here 

 we can see the beginnings of the segmentation and inflation of the 

 joints, which constitute the x^erfected structure. The manner of for- 

 mation of these segments has not been studied as yet, but the general 

 theory of the origin of the entire structure is probably the same in 

 this case as in others in the animal kingdom. The violent vibrations 

 into which most snakes throw their tails when excited has determined 

 nutritive processes to its extremity and produced the excessive growth. 



The species of this genus are of rather sluggish movements, and are 

 not quick to bite unless trodden on. They throw the body into a coil 

 and sound the rattle, giving a sigmoid flexure to the anterior part of 

 the body, on which the head is poised with open mouth ready for 

 action. At this time droits of the poisonous saliva fall from the fangs, 

 and by a violent expulsion of air from the lungs are thrown at their 

 enemy. In the act of biting the movement is threefold. First, tlicre 

 is the spring of the body, which never exceeds two-thirds of its lengtli ; 

 second, the bite proi)er, caused by the seizing by the jaws; and, third, 

 the clutch with the fangs themselves, which are moved freely back- 

 ward and forward by the flexor and extensor muscles of the maxillary 

 bone on tlie prefrontal as a fixed point. This grasping movement may 

 be observed in Crotali when very much excited and anxious to bite, 



