46 OPHIDIANS. 
The number and arrangement of the non-imbricated shields 
of the head are as follows: 
Rostral. Ocular anterior (or preorbital). 
Anterior frontal. Ocular posterior (or postorbital). 
Posterior. Upper labials. 
Vertical. Temporals. 
Supraciliary. Mental. 
Occipital. _ Lower labials. 
Nasals. Chin shields. 
Loreal. 
The heart has three cavities, one ventricle, and two auri- 
cles; the urinary bladder is wanting. They are all carniy- 
orous and swallow their prey entire; are oviparous, ovo-vivi- 
parous, or viviparous, and hematocrya or cold-blooded. Their 
‘blood is rich in solid constituents, and has red corpuscles of 
an elliptical shape, flattened, biconvex, and smaller than in 
other reptilia. Of the 21 genera already classified, 4 only 
have been considered poisonous; but in South America 
varieties occur under the genera Calamiris, Homalopsis, and 
Acrochordus, which are known beyond a doubt to be poison- 
ous; and some, species, which must be classified under the 
genera Python and Typhlope, found in India, are also ven- 
omous. 
This divides the 21 genera into 
Non-venomous, ; : ; 12 genera. 
Venomous, .. ' 9 genera. 
The most deadly of these, viz., Ophiophagus elaps and Naja 
tripudians belong exclusively to India, whilst the Viperide 
are represented by the Daboia Russellii, and the Crotalide by 
the Trimeresuri. The latter genus is less venomous than the 
Crot. horridus of North America; the Crot. cascabellus ; the 
