38S 



Abastor, Virginia, Contia, Pituophit, Cenckris, Crotalophorxu, 

 Uropsophus, Cro talus. 



Forma common with other regions. — Heterodon, Coluber, Cory- 

 photlnn, Herpetodryas, Cyclopias, Elans. 



There is some difficulty in stating the southern boundary of this 

 region ; the Tropical fauna advances along the Isthmus of Panama, 

 and extending over the again expanding part of Southern Mexico, ii 

 is gradually mixed with the Arctic fauna. And in these parts tie- 

 fauna of the same latitude is the more mixed on account of the great 

 differences of the elevation above the level of the >ea, and the result- 

 ing great variety of climate in a small space ; but as the climate gra- 

 dually assumes the tropical character, so do also the vegetative and 

 animal life. Nevertheless we have in the New World two quite dif- 

 ferent creations, radiating from the system of the Mississippi in the 

 north, and from that of the Amazon in the south ; and in each of 

 those smaller provinces situated on the boundary between both regions, 

 it will be a question, whether the larger number of its species belongs 

 to northern or southern forms. As far as we are able for the pre- 

 sent to judge, the tropic of Cancer may be considered as the bound- 

 ary. No Snake is to be found north of the GO N. Int., in a lati- 

 tude where in the Palsearctic region Pelias berus exists. But taking 

 6,500,000 square miles as the amount of the whole dry land in this 

 region, and allowing seventy-five* species as peculiar to it, we have 

 one species to every 87,000 square miles, or four species to the same 

 area, for which we found only one in the Palsearctic region. Thus 

 this region indicates a much greater degree of intensity of species 

 than the Palsearctic region ; but if it be stated that it surpasses also 

 the ^Ethiopian region, this I consider as not an established fact, but 

 only an appearance caused by the circumstance that North America 

 has been much more fully explored than Africa. Even then, if we 

 consider (according to Dr. Gray's system) Charina and Wenona to 

 be Boidte, the ratio of this section to the number of Colubrina is very 

 small (1 : 18), the ratio between Viperina ami Colubrina being large 

 (1 : 5) ; in this respect this part of the fauna quite agrees with the 

 same part of the Old World. 



Among the non-venomous Colubrina the two families of Calama- 

 rieUe and Natricidxe offer the most generic and specific forms. The 

 type of Heterodon is a North American form ; but there is al-o one 

 species to be found in South America. 



Colubrina with grooved fangs in front can hardly be considered a- 

 pertaining to this region, only two species of Elaps reaching into the 

 most southern parts. The Viperine Snakes are represented by most 

 peculiar forms, all belonging to the family with a pit 00 each side of 



* Without ramming up the numl.fr of nil the North American ipec et de- 

 scribed nnce the publication of the * Catalogue of North American R 

 Baird and Girard, L853, I only mention that they describe therein I ID ip 

 Wh.it I think of sin h ipeciea u shown by the synonymy of the North taarieaii 

 Snakes in my catalogue. 



No. CCCLXXI. — Proceedings oi rm Zoologicai Bocim 



