v j PREFACE. 



tissues, sometimes to a considerable degree ; and, although the author 

 has been ever mindful of such an occurrence, his pen may have erred 

 in sundry details of forms or outlines, as well as in the coloration. 



When comparisons with genera and species allied to those described 

 appear deficient, the cause of such deficiency is want of materials. 



Of the order of Batrachians, the principal groups are represented 

 in the collection ; thus enabling the author to institute comparative 

 studies of most of the genera. 



Not so with the Ophidians ; many links are unconnected. Hence, 

 the impracticability of arranging them into natural families. There 

 are even genera the limits of which could not be delineated ; for there 

 is no general collection of exotic serpents in the United States, to 

 which access might have been had, in order to extend the foregoing 

 researches and comparisons. As a general rule, it has been deemed 

 inexpedient to compile from the writers in the same field, where no 

 specimens were available, the author preferring to record in an unpre- 

 tending form, whatever observation he might be able to make. 



Amongst Saurians we miss, likewise, representatives of several fami- 

 lies. The difficulties encountered in their investigation have proved 

 greater still, and the gaps wider yet, than for the Ophidians. 



As to the Chelonians, the materials at command were but few and 

 fragmentary, and for the determination of them, the author has 

 unexpectedly been led into inquiries of a purely historical character, 

 and which, it is assumed, have thrown a new light upon the entire 

 subject of Marine Turtles or Tortoises. 



A general glance at the Reptiles, which we have thus been review- 

 ing, presents two main features : one portion of the collection apper- 

 taining to Continental, the other to Pelagic Faunas. 



The representatives of Continental Faunas were obtained in the 

 neighborhood of the various ports touched at by the Expedition : the 

 coasts of Brazil, Patagonia, Chili, Peru, New Holland, Bengal, and 

 Northwestern America. The others, in the coral and volcanic islands 

 of the Pacific Ocean. 



A curious fact in the geographical distribution of these animals, on the 



