682 NORTH AMERICAN SNAKES COPE. 



plate aud caudal scuta uudivided. No rattle. Body aud tail cylin- 

 dric. 



Three species of this genus are known, two of which belong to the 

 Nearetic, and one to the northern jiart of the Neotropical Eeahn. They 

 are snakes of robust habit, and their bite is highly dangerous. One is 

 terrestrial in habit, and the other semiaquatic. They differ as follows: 



I. No loreal ; two small plates behiud the parietals ; eye resting ou labials. 



Scales in twenty-five rows ; brown wnth broad blackisb-brown cross bands with 

 zigzag borders, and the lateral centers pale and with a median dark 

 spot ; a light stripe from superciliary plate aud one from below eye, which 



reaches labial border of last upper labial A inscivorus. 



.II. Loreal present; plates behind parietal rudimental; eye separated from labials by 

 scales. 



Scales in twenty-five rows ; dark grayish with brown crossbands, wide on the 

 middle line, and with imperfect yellow borders; belly black, with trans- 

 verse yellow spots on the sides; a yellow stripe from end of muzzle and 

 superciliary plate; other yellow stripes on borders of rostral plate and 

 through centers of superior labials A. hilineatus.^ 



Scales in twenty-three rows; grayish with copper-colored cross bands, much 

 narrowed on the middle line, and with pale centers laterally ; no stripes ou 

 head, but a color border from middle of orbit to top of last upper labial; 

 belly whitish, with black spots on sides •. A, contortrix, 



Baird and Girard have proposed to separate the A. piscivorus as 

 type of a genus Toxicophis, on account of the presence of a pair of post- 

 parietal scuta and the absence of loreal. The A. bilineatus is, however, 

 intermediate between that species and the A. piscivorus in having 

 traces of the postparietals and a loreal plate. 



Several characters are common to the species of Ancistrodon which 

 are also found in other genera of Crotalida?. As in all genera with 

 scuta on the top of the head, the superior plane of the muzzle makes 

 a right angle with the sides, forming a strong canthus rostralis, which 

 is continued round the apex of the rostral plate. The fossseof the epi- 

 dermal scales are situated farther back than in the genera of the harm- 

 less snakes, and a small tuberosity of the true scale fits into each of 

 them. There are frequently several divided urosteges near the end of 

 the tail, but their number is irregular, and they are sometimes absent, 

 as in Ancistrodon contortrix No. 10361. The last of the caudal verte- 

 brte consists of an osseous splint with acute apex, which is ensheathed 

 in three modified scales, two above and one below, which is better de- 

 veloped thau in most harmless snakes. This process is thrown into 

 rapid vibration when its possessor is alarmed, and produces a buzzing 

 sound when among dry leaves or other objects. It foreshadows the 

 rattle of the Crotali. It is especially developed in the Neotropical 

 genus Lachesis. It is v^ariable in size in the Ancistrodons. 



*Ancistroi><)X B1L1NEATU.S Giiuther, Ann. Magas. Nat. Hist. 1863, November. Tlie 

 U. S. National Museum i)osscsses two specimens I'tomTehuantepec (west side), Mexico, 

 from Francis Sumichrast. TLe largest is equal to an average sized Ancistrodon 

 contortrix. 



