PYTHONS AND BOAS. 189 



head usually has a more or less distinct streak behind the eye ; while the under- 

 parts are pale olive or yellowish, more or less spotted with brown or black. 



Dog-Headed Closely allied to the last, the five species of the genus Corallus 



Tree-Boa. are distinguished by having deep pits in the labial shields of both 

 the upper and lower lips. The body is compressed, with small smooth scales, and 

 the prehensile tail is either short or more or less elongated. This genus has a 

 somewhat remarkable distribution, four of its representatives being inhabitants 

 of tropical America, while the fifth (C. madagascariensis). which is distinguished 

 from the rest by the shortness of its tail, is restricted to Madagascar. The 

 dog-headed tree-boa (C. caninus) is a native of the Guianas and Brazil, and 

 usually attains a length of some 5 feet, although it may be considerably larger. 

 It belongs to a group of two American species distinguished from the other kinds 

 inhabiting the same countries by the relatively shorter tail, which has only from 

 sixty-four to eighty -two shields on its inferior surface; whereas in the true 

 tree-boa (C. hortulanus), and another species, there are at least a hundred of 

 these shields. The species here figured is specially characterised by having the 

 scales arranged in sixty-one or seventy-one rows, and by the number of shields on 

 the under surface of the body ranging from one hundred and eighty-eight to two 

 hundred and nineteen, while those on the tail vary from sixty-four to seventy-nine. 

 In colour this snake is decidedly handsome, the upper-parts of the adult being 

 bright green, ornamented with irregular spots and crossbars of white, and the 

 under-parts bright yellow. In the young the ground-colour is yellowish, and the 

 white markings are edged with dark green or purplish black. Most abundant 

 in the neighbourhood of the Amazons, this species becomes more rare in Guiana, 

 while southwards it likewise diminishes in numbers in lower Brazil. Feeding 

 principally upon birds, the dog-headed boa is an excellent swimmer, and has been 

 observed both in the Bio Negro and in the salt-water of the beautiful harbour 

 of Rio de Janeiro. Although it frequently visits the huts of the Brazilian negroes 

 in search of prey, it does not appear that this snake ever voluntarily attacks 

 human beings. If, however, it is driven to bay and unable to escape, it is capable 

 of inflicting very severe bites with its long front teeth, such wounds being 

 difficult to heal. 



Keeled A third genus of tree-boas (Enygrus) is distinguished from both 



Tree-Boas, the preceding by the scales having distinct keels ; the labial shields 

 of the head being devoid of pits, and the tail short and prehensile, with a single 

 row of shields on its inferior surface. This genus is represented by four species 

 inhabiting the Moluccas, the Papuan region, and Polynesia. 



This gigantic snake is the sole member of a group of several 

 genera, distinguished from the tree-boas by the teeth gradually 

 decreasing in size from the front to the back of the jaws without any marked 

 enlargement of those in the fore-part. Merely mentioning the allied tropical 

 American genera, Trachyboa, Ungalia, suadUngaliophis, the first and last of which 

 are each represented only by a single species, we may observe that the anaconda 

 is specially distinguished as a genus by the large size of the rostral shield of 

 the head, behind which one pair of the nasals come in contact with one another 

 in the middle line, and by the very small size of the smooth scales of the body. 



