202' rniLORoriiY of zoology. 



Hinder part of tlie body and tail much compressed. 

 This form enables the species to swim with ease. They fre- 

 quent arms of the sea, lakes, and rivers. Head covered 

 with large plates. 



48. Pelamis. Hind head swollen. Scales rather reti- 

 culated than imbricated. Ventral shields minute. P. bicolar. 



49. Hydrophus. Head not swollen. The shields are 

 more distinct than in Pelamis. H. ayspisurus. 



S. Upper maxillary bones destitute of common teeth, 

 supporting only the poison fangs on each side. 



The shields entire on the belly, and under the tail beyond 

 the vent. 



a. The entire shields under the tail continued to the ex- 

 tremity. 



Tail furnished with a rattle, (crepitaculum.) 



This very remarkable organ occurs as an appendage to 

 the tail, the last vertebra of which is enlarged to serve as a' 

 mould for its production. This vertebi*a is somewhat coni- 

 cal, rounded at the extremity, subquadrangular, with three 

 rounded circular ridges, the largest of which is next the 

 body. The first formed cup of the rattle, which, in sub- 

 stance, is similar to the scales, exhibits the form of the verte- 

 bra, from which, however, it is separated by a thin mem- 

 brane. It is connected directly with the scales on the back , 

 and, by the intervention of a row of small scales, with the 

 shields on the belly. Thus formed, the cup has three ridges 

 externally, with corresponding grooves internally. 



When this first cup has arrived at maturity, a second 

 begins to form underneath. The terminal and middle 

 ridges of the second grow within the grooves of the middle 

 and basilar ridges of the first. As these ridges of the 

 second increase in size, they push from the vertebra the 

 first formed joint, and the second cup appears with its basi- 

 lar ridge, only visible externally, its middle and terminal 



