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 an , difierence 

 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 LacepMe, 1789 (Histoire des 

 Serpens, II. pp. 390-420, PI. 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, 

 PI. XII.). For comparison with what has been recorded above, a few 

 sentences are quoted from Lacepede (p. 404). Speaking of the mode of 

 growth, he says : " Quand une piec>3 est formee, il se produit au-dessous 

 une nouvelle piece entierement semblable a I'ancienne, et qui tend ^ 

 la detacher de I'extremitc de la queue. L'ancienne pi^ce ne se separe 

 pas cependant tout-k-fait du corps du serpent ; elle est seulement re- 

 poussee en arriere ; elle laisse entre son bord et la peau de la queue, un 

 intervalle occup^ par le premier bourlet de la nouvelle piece ; mais elle 

 enveloppe tou jours le second et le troisieme bourlets de cette nouvelle 

 piece, et elle joue librement autour de ces bourlets qui la retiennent. 

 . . . Si les dernieres vertebres de la queue n'ont pas grossi pendent 

 que la sonnette s'est formee, chaque piece qui i'est moulee sur ces ver- 

 tebres a le memo diamctre ; et la sonnette paroit d'une ^gale largeur 

 jusqu'k la piece qui la termine ; si, au contraire, les vertebres ont pris 

 de I'accroissement pendant la formation de la sonnette, les bourlets de la 

 nouvelle piece sont plus grands que ceux de la piece plus aucienne, et le 

 diametre de la sonnette diminue vers la pointe." 



