MORPHOLOGY OP VENOMOUS SNAKES 35 



A. bibronii, length 2 feet; east of Cape Colony; Natal, Namaqualand, 



Angola. 

 A. aterrima, length about 2 feet; central and western Africa. 

 A. dahomeyensis, length about 1.5 feet; Dahomey. 

 A. micropholis, length about 1.2 feet; Cape Verde. 

 A. leucomelas, length about 2 feet; Somaliland. 

 A. microlepidota, length about 1.75 feet; western and central Africa. 



Subfamily CROTALIN^ Boulenger. 



The essential difference of this subfamily from the Viperinae is the presence of 

 a deep cavity or pit between the eye and the nose, lodged in the hollowed-out maxil- 

 lary bone. This pit is lined with a modified continuation of the epidermis, and 

 is amply supplied with branches from the trigeminal nerve. It is undoubtedly 

 sensory, but we do not know its function.^ All snakes belonging to this group 

 are called "Pit-vipers." 



The maxillary bone, into the lower end of which the large hollow fang is immov- 

 ably fastened like a knife in a handle, is extremely shortened and higher than long, 

 so as to appear to be in a vertical position. On the other side of this bone there is 

 the deep cavity which separates two articular surfaces. The upper surface of the 

 maxillary forms with the corresponding concave face of the prefrontal (lachrymal) 

 bone, which projects from and articulates with the frontal bone, a hinge-like joint 

 allowing considerable freedom of motion. The lower articular surface receives 

 the flattened anterior end of the external pterygoid bone (transversum) . If the 

 ectopterygoid be moved forward or backward, the maxillary hinges on the prefrontal, 

 and if the same is pushed forward the fang is erected. 



The Crotalina^ are divided into two groups according to whether the snake has 

 the "rattle" or not. The rattleless snakes fall into two genera, Ancistrodon and 

 Lachesis; the rattlesnakes are divided into Sistrurus and Crotalus. About 60 

 species are known. 



The rattlesnakes are restricted to America, but the rattleless Crotaline snakes 

 are found in North and South America and the southeastern half of Asia. No pit- 

 viper is found in Africa, Australia, the southwestern corner of Asia, and Europe, 

 except one species which enters the extreme southeastern corner. 



Genus ANCISTRODON Beauvois. 



Originally transliterated as Agkisirodon, dyKicrrpov, hook, dSuv, tooth. 



^ Without a ratde. Sensory pits between nose and eye. Surface of head covered 

 with 9 large shields, sometimes broken into small scales. Body cylindrical, covered 

 with smooth or keeled scales which have special fossa. Tail moderate or short, 

 subcaudal scales in one or two rows. From northern borders of the Caspian Sea, 

 throughout most of the Asiatic mainland and in North and Central America. In 

 the Asiatic mainland to Himalaya in the south and to Lake Baikal in the north. 

 Japan (Formosa and adjacent islands) has some representatives. 



Ancistrodon piscivorus Lac^plde. (Plate 15, b; plate 16, A.) 

 Trigonocepltalus piscivorus Holbrook. 

 Crotalus piscivorus Lacepfede. 

 Cenchris piscivorus Gray. 

 Toxiophis piscivorus Baird andGirard. 

 Coluber aquaticus Shaw. 



The water-moccasin or "cotton mouth" has a round muzzle. Scales of body 

 keeled, in 25 rows, subcaudals in one row, ending in two rows towards the point. 

 General color dark chestnut-brown, with darker markings. Head above purplish- 

 black. On each side a series of 20 or 30 narrow, vertical, purplish-black bars i or 



1 A good anatomical account by West, Trans. Linn. Soc, XXVIII. 



