334 PROCEEDINGS OP THE ACADEMY OP 



In the foetal Crotalus the gastrosteges are divided, but they unite into broad 

 shields some time before the young animal bursts its membrauous envelope. 

 But it is not until some size is attained that the grooved line where the grow- 

 ing edges of the fang unite and isolate the canal, is obliterated. 



Thus we see typified the three families of the venomous serpents, and their 

 relative positions in the scale of being. But these characters, while parallel 

 to those deduced from the skeletal structure, do not, like them, define the 

 groups they characterize. ' 



Those genera of serpents which are characterized by entire urosteges, ex- 

 hibit a degree of development one step beyond those that have them divided, 

 since the latter retain that arrangement which characterizes the foetal condi- 

 tion of the former. 



I. VIPERIDiE. 

 1. CROTALINJE. 



Fangs without external groove. Pupil erect, elliptical. Head very distinct. 

 A deep fossa on each side behind the nostrils, partly occupying the excavated, 

 superior maxillary bone. 



The genera of this group are here arranged in what appears to be their 

 natural succession, without reference to the position of that which exhibits the 

 highest typical perfection. 



Total species Species in 

 estimated. Mus. Acad. 



A. Vertex symmetrically plated. 



* Urosteges two-rowed. 



Numerous scale-like plates substituted for the frontals, 



1. Hypnale, 

 Frontals, two pairs, regular. 2. Trigokocephalus, 



** Urosteges, one-rowed. 

 Tail terminating in a horny point. 3. Ancistkodon, 

 Tail terminating in a rattle. 4. Crotalophorus, 



B. Vertex, scaled. 



* Urosteges, one-rowed. 



Tail terminating in a rattle. 5. Crotalus, 



Tail without rattle. 6 . Teleuraspis, 



** Urosteges, two-rowed. 



f Superciliary region p'lated. 



Urosteges four-rowed at the tip. 7. Lachesis, 

 Urosteges two-rowed to the tip ; soales carinated. 



8. Bothrops, 

 Urosteges two-rowed to the tip ; scales smooth. 



9. Parias, 

 Superciliary plates, two pairs, 10. Meg^ra, 



ft Superciliary region scaled. 

 A rudimental superciliary resting on the preoculars. 



11. Cryptelytrops, 

 No rudimental superciliary plate. 12. Atropos, 



2. VIPERIN^E. 



Fangs without external groove. Head very distinct. Pupil usually ellipti- 

 cal. No lachrymal fossa. 



A. Superciliary region scaled. 



* Urosteges two-rowed. 



TDec. 



