1132 REPORT OF NATIONAL MUSEUM, 1898. 



Nine symmetrical plates on top of the Lead, the superciliary bound- 

 ing the orbit above. I^asal plates, two. Scales keeled, bifossate. 

 Anal plate and caudal scuta undivided. 'No rattle. Body and tail 

 cylindric. 



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

 iNearctic and one to the northern part of the Neotropical realm. They 

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

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



I. No loreal; two small plates l>elnii(l tlie parietals; eye resting on labials. 



Scales in 25 rows; brown, with broad lilaokisli-brown cross-bauds with zigzag 

 borders, and the lateral centers ])ale and with a median dark spot; a light 

 stripe from snperciliary ))late and one from below eye, which reaches labial bor- 

 der of last Tipper labial A.j)iscirorus. 



II. Loreal present; no plates behind parietal; eye separated from labials by scales. 

 Scales in 2.5 rows; dark grayish with brown cross-bands, wide on the middle 



line, and with imperfect yellow borders; belly black, with transverse yellow 

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

 and thin yellow stripe on borders of rostral plate and through centers of 



superior labials A. hilineatiis.^ 



Scales in 23 rows; grayish, with copper-colored cross-bands much narrowed on 

 the middle line, and witii pale centers laterally; no stripes on head, but a 

 color border from middle of orbit to top of last upper labial; belly whitish, 

 with black spots on sides A. coniortrix. 



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

 type of a genus To.vicophh, on account of the ])resence of a ]iair of post- 

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

 intermediate between that species and the ^4. contortrix in having 

 traces of the posti)arietals 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 con- 

 tinued around the apex of the rostral i)late. The fossie of the epider- 

 mal scales are situated farther back than in the genera of the harmless 

 snakes, and a sunill tuberosity of the true scale fits into each of them. 

 There are iVe(iuently several divided urosteges near the end of the tail, 

 but their number is irregular, and they are sometimes absent, as in 

 Ancistrodon contortrix, Cat. Xo. 103G1. The last of the caudal vertebra^ 

 consistsof an osseous splint with acute apex, which is ensheathed in three 

 modified scales, two above and one below, which is better developed 

 than in most harmless snakes. This process is thrown into rapid vibra- 

 tion when itsi)ossessoris 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 variable in size in the Ancistrodons. 



Uncistrodon hilineatus Giinther, Ann. Mag. Nat. Hist., (3 ) XII. 1863, p. 364. The U. S. 

 National Museum possesses two specimens from Tehuantepcc (west side), Mexico, 

 from Francis Sumiclirast. The largest is eijual to an average sized Ancinirodou con- 

 tortrix. 



