Hurter — Herpetology of Missouri. 203 



one, for not only does it indicate the relationship of these 

 snakes to the true vipers, but it also contains a reference 

 to the remarkable character which at once distinguishes 

 them both from the vipers and all other snakes as well. 

 The name refers to a deep pit or hole found in the Rattle- 

 snakes and their nearest relations on the side of the face 

 between the nostril and the eye. This cavity sinks deep 

 into the maxillary bone and represents a "blind" sac 

 lined with epidermis and is not connected with any of the 

 other cavities or organs in the head by any inside opening 

 or canal. There is nothing similar to be found in any 

 known reptiles outside of this family, if we except the 

 labial pits in the pythons and boas, nor is there any in 

 any other class of animals. 



' ' In the meantime naturalists have become compelled to 

 assume the existence of a ' sixth sense ' in various animals, 

 for which they had discovered special sense organs, such 

 as, the lateral line in fishes. 



"It was quite natural, then, that Prof. Leydig should 

 come to the conclusion that the pit of the Crotalidae is the 

 organ of a sixth sense, when upon a microscopic examina- 

 tion of the pit's lining he found it supplied with a thick 

 nerve, ending in a way the only analogue of which is 

 found in the retina of the eye or the labyrinth of the ear. 



"The external layer of the lining of the pit Leydig 

 found to be a continuation of the outer skin, which, how- 

 ever, upon entering the cavity becomes thin and consid- 

 erably modified. The granular tubercles gradually disap- 

 pear toward the bottom, and the surface is found to be 

 composed of large angular epidermis plates containing 

 nuclei. Underneath this he found a layer of connective 

 tissue, in which the fine ramifications of the thick nerve 

 supplying the pit are lost in a granular substance which 

 under high power reveals itself as containing numerous, 

 true, rounded, but pale nuclei. The granular substance 

 he found arranged around the nuclei in such a way as to 

 form groups or islands of various forms and sizes sep- 

 arated by light narrow spaces. These structures can only 



