MORPHOLOGY OF VENOMOUS SNAKES 35 
. bibronii, length 2 feet; east of Cape Colony, Natal, Namaqualand, 
Angola. 
. aterrima, length about 2 feet; central and western Africa. 
. dahomeyensis, length about 1.5 feet; Dahomey. 
. micropholis, length about 1.2 feet; Cape Verde. 
. leucomelas, length about 2 feet; Somaliland. 
. microlepidota, length about 1.75 feet; western and central Africa. 
BRR Rw DB 
Subfamily CROTALINZ Boulenger. 
The essential difference of this subfamily from the Viperine 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.t All snakes belonging to this group 
are called “ Pit-vipers.” 
The maxillary bone, into the lower end of which the large hollow fang is immoy- 
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 pretrontal, 
and if the same is pushed forward the fang is erected. 
The Crotaline are divided into two groups according to whether the eke 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 Agkistrodon, dyx.oTpov, hook, 65wv, tooth. 
Without a rattle. Sensory pits between nose and eye. Surface of head covered 
with g 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 Lacépéde. (Plate 15, B; plate 16, A.) 
Trigonocephalus piscivorus Holbrook. 
Crotalus piscivorus Lacépéde. 
Cenchris piscivorus Gray. 
Toxiophis piscivorus Baird and Girard. 
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 1 or 
1 A good anatomical account by West, Trans. Linn. Soc., XXVIII. 
