294 PROCEEDINGS OF THE ACADEMY OP 



The regular aggregation and deepness of these punctulations, form a series of 

 about forty-two dorsal spots. These are transverse, with produced lateral 

 angles, extending across twelve rows of scales from angle to angle, separated 

 from the adjacent ones by a bright band of ground color one and a half scales 

 wide. On the posterior fourth of the total length, they form brown cross 

 bands : five upon the tail are black on a very light ground as in C. atrox. 

 Anteriorly there is an ill-defined series of spots which are opposite those of the 

 dorsal line. A yellow band extends from the nasal plates anterior to the eye, 

 involving from the ninth to the last superior labial. Superior to this is a 

 brown band extending from the eye and ceasing on a line with the angle of 

 the mouth. Some indistinct brown marks on tke top of the head are arranged 

 as follows : one on the inner border of each superciliary ; three posterior to 

 these, the median short and broad ; four further posterior, the median 

 pair longer, diverging, reaching the neck. Cape St. Lucas, Lower Cali- 

 fornia ; one specimen (5291 j Sm. No.) in Mus. Smithsonian from Mr. John 

 Xantus. 



This curious rattlesnake is related to C. t i g r i s, C. cerastes, and C. 

 lucifer. In common with the first two and C. e n y o, lepida and 

 molossus, it exhibits a low rostral plate. The plates of the superior parts 

 of the muzzle resemble only those of (among the above mentioned species) 

 cerastes and enyo, being small, irregular and rough, without even the 

 marginal series seen in 1 u c i f e r, atrox, h o r r i d a, etc. In shade of color- 

 ation it is not unlike tigris, being well adapted for concealment upon the 

 sandy soil of the Californian deserts : the distribution and form of the spots 

 are like those of 1 u c if e r. The separation of the prenasal from the rostral 

 plate is peculiar to the species. It is named in honor of Dr. S. W. Mitchell, 

 the author of the interesting " Researches upon the Venom of the Rattlesnake." 



In the catalogue of rattlesnakes in the Smithsonian contributions, pre- 

 viously cited, thirteen species of the genus Caudisona were referred to, as dis- 

 tinguishably described. Two have been since added to this list, making, with 

 those of the present memoir, the whole number seventeen. Of these, three 

 inhabit South America, six Mexico, two Lower California, and eleven the 

 United States. Two of the eleven are found east of the Mississippi River ; one 

 west of the Rocky Mountains ; the intermediate region is inhabited by ten 

 species, 1 u c i f e r entering from the west, and h o r r i d a from the east. Of 

 these, the most northern, and widely diffused isLe C o n t e i ; it extends from 

 southern Nebraska to Utah ; in the great basin of the latter country a curious 

 variety of it is found. C. a t r o x alone inhabits the greater part of Texas ; in 

 the extreme west of that State, and probably in Chihuahua, C. lepida 

 occurs. The greatest intensity of species is in south western New Mexico 

 and Apacheria (or Arizona), where are found tigris, cerastes, scutu- 

 latus, atrox sonoraensis, molossus, and perhaps lucifer. 



Structurally, the South American species and molossus form a group 

 characterized by the six regular plates of the muzzle, and the small rattle. 

 The single nasal and smooth head plates isolate the 1 e p i d a. The superci- 

 liary hornlike processes, and the rostral plate, broader than high, separate the 

 cerastes. The remaining species form the largest group, where there are 

 two nasals, one or more pairs of marginal plates between the superciliary and 

 rostral, separated on the median line by smoother or rougher, small irregu- 

 lar scales ; no superciliary processes. C. mitchellii must be distinguished 

 from these by its absence of marginal plates, and presence of scales on the 

 lateral borders of the rostral. All the species have Professor Reinhardt's 

 scale pores in pairs ; they are very difficult to observe in some of the species 

 as cerastes and mitchellii. In durissa, a single pore is frequently 

 met with. 



Bothriechis mexicanus Cope. Atropos 3Iexicanus, Dum. Bibr. vii. p. 1521 . 

 Specimens in Mus. Smithsonian and Academy from Dr. C. Sartorius from 



[Sept. 



