My Dear Sir

It gives me great pleasure to answer in so far as I am able your enquiries with regard to the rattlesnake.² Dumeril & Bibron have given in their Herpetol. Generale,³ a somewhat confused description of the structure of the rattle & Czermack has published a memoir entitled “Uber den schallenzeugenden Apparat von Crotalus” Zeitsch. f. wissench. Zool. Vol VIII 1856.⁴ This last I have not seen, & only know it through the short account taken from it in Van der Hovens Zoology—⁵ As I have some good specimens of tails in alcohol, at the risk of sending you superfluous information, I shall give you some account of the examination of them, which I have been led to make by the questions contained in your last letter.

The first specimen is that of a foetus of Crot. durissus which was nearly mature, is nearly eight inches long, & was taken from the body of the parent. For this I am indebted to Prof Baird of the Smithsonⁿ. Institution.⁶ On the tail of the foetus one rattle is already formed, fits the end of the tail very closely & has a somewhat different shape from the subsequent ones in the same species. It is a true cuticular covering & is continuous with the cuticle on the rest of the body.

The other specimens are from adults of C. rhombifer, & are fortunately in a condition to show very well one stage in the formation of the rattle. I will state first of all, that the last three caudal vertebrae are very much compressed laterally, anchylosed, & covered with a somewhat thickened layer of integument. The body thus produced has three prominent ridges as at a in the figure, & is the matrix on which the rattle is formed.

In two instances a new joint or shell of the rattle has just been completed, but still fits tightly to the matrix in every direction, to which it sustains the relation of common cuticle. When mature the covering thus secreted becomes detached from the matrix over its whole surface, & is forced back in such a manner that it now grasps only the second & third ridges instead of covering all of them. In this way the free space between it & its successor is gained, as at b & c   I have never had an opportunity of witnessing the moulting of one of these snakes & cannot therefore describe the process by which the new formed joint is thus forced back— I suppose that in the first instance there is a shrinking of the matrix by which the rattle is detached & then, as each successive rattle is larger than its predecessor, the growth of the matrix which follows the separation pushes the rattle back. However this may be, one direct observation will be worth more than any or all suppositions. That the slipping of the rattle does take place I am sure, for numerous specimens which I have examined show it.

You mention the habit which the Trigonocephalus has, of vibrating its tail.⁷ Perhaps it will be no news to you, but I will mention at a venture, that our common blacksnake (Coluber constrictor) has the same habit, & when its tail is made to vibrate against dried leaves, the sound which is produced is quite startling from its resemblance to that of the rattlesnake. The sound of the blacksnake I have heard— I am also informed by others, among them by Prof Baird that there are several species in our western country which have the habit as well.