MUSEUM OF COMPARATIVE ZOOLOGY. 263 
shapes, as in sizes, and there are also considerable differences in the 
rattles within the species. In the case of the small snake Crotalus 
exsul, Garm., from Cedros Island, Lower California, the large size of 
the first ring is evidence of derivation from a larger species, probably 
C. lucifer, B. & G., of the mainland. In this case, the change in button 
has not kept pace with the reduction in size of body, or the changes in 
squamation, etc. While the rings vary with rapidity of growth in the 
body, from amount of food, it is unlikely that it makes any difference 
in their number, or that of the sloughings. 
More than seventy specimens have been looked over for evidence of 
growth of a new button between the months of May and September ; 
two, and a doubtful third, favor the conclusion that a ring is added in 
the fall. One of these, as it was in September, is.sketched in Figures 
5 and 6. On the other hand, living specimens kept through the winter 
prove that a new growth does take place toward spring, and that when 
the epiderm is shed, on coming out of winter quarters, the animal is 
possessed of an addition to the rattle. 
The mechanism of the organ has been so often described and figured 
that it is unnecessary to give a detailed description here. Among the 
most accurate of the earlier writers is Lacépéde, 1789 (Histoire des 
Serpens, II. pp. 390-420, Pl. XVIII.); and of the more recent, Czermak, 
1857 (Ueber den schallerzeugenden Apparat von Crotalus, in Vol. 
XIII. of the Zeitschrift fiir wissenschaftliche Zoologie, pp. 294-302, 
Pl. XII.). For comparison with what has been recorded above, a few 
sentences are quoted from Lacépéde (p. 404). Speaking of the mode of 
growth, he says: “‘ Quand une piece est formée, il se produit au-dessous 
une nouvelle piece entierement semblable 4 l’ancienne, et qui tend a 
la détacher de l’extrémité de la queue. L’ancienne piece ne se sépare 
pas cependant tout-a-fait du corps du serpent; elle est seulement re- 
poussée en arriere ; elle laisse entre son bord et la peau de la queue, un 
intervalle oceupé par le premier bourlet de la nouvelle piece ; mais elle 
enveloppe toujours le second et le troisitme bourlets de cette nouvelle 
piece, et elle joue librement autour de ces bourlets qui la retiennent. 
Si les derniéres vertébres de la queue n’ont pas grossi pendent 
que la sonnette s’est formée, chaque pitce qui s’est moulée sur ces ver- 
tébres a le méme diamétre ; et la sonnette paroit d’une égale largeur 
jusqu’a la piéce qui la termine; si, au contraire, les vertébres ont pris 
de l'accroissement pendant la formation de la sonnette, les bourlets de la 
nouvelle piéce sont plus grands que ceux de la piéce plus ancienne, et le 
diamétre de la sonnette diminue vers la pointe.” 
