BOTHREMYS. m 



The most remarkable character of the skull is seen in the inferior view (Fig. 7). 

 This consists of a deep conical pit (a) occupying each maxillary bone. The bottom 

 of the pit extends as high as the position of the upper third of the orbit externally, 

 and is imperforate. The pit expands regularly downward upon nearly the whole 

 palatine surface of the maxillary bone, including a portion of the premaxillary and 

 palate bones. The lower boundary, or mouth of the pit as it may be considered to 

 be, forms an isosceles triangle, the apex of which, sUghtly prolonged, extends upon 

 the premaxillary bone to the median palate suture, and the base is formed by the 

 posterior, acute, crescentic border of the maxillary bone bounding the temporal fossa. 

 The outer side of the triangle is constituted by the dentary margin of the jaw ; 

 and the inner side is formed by a ridge proceeding from the middle of the premax- 

 illary bone obliquely outward and backward, just external to the posterior nares, 

 and extending upon the palate bone. No mtervening ridges occupy the triangle 

 formed by the mouth of the pit. 



The function of the latter it is difficult to comprehend. It does not appear like 

 an alveolus for a tooth ; but probably it may have accommodated a corneous tooth- 

 like process springing from a corresponding hoUow of the lower jaw. 



The posterior nares are situated about ten lines behind the front of the jaw. 

 They are circular, are separated by a strong osseous vomer, and are directed back- 

 ward in the usual manner. 



The base of the skull between the temporal fossae presents a broad surface, which 

 was apparently furnished at the sides with conspicuous pterygoid processes, as in 

 the great Turtle of the Amazon. 



A pair of small anterior palatine foramina occupied the suture between the pre- 

 maxiUaries, and apparently the vomer and palate bones. 



A rather larger pair of posterior palatine foramina occupy the sutures between 

 the palate and ali-sphenoid bones. 



The orbits are separated from the temporal fossae, as in the great Turtle of the 

 Amazon, by a postero-external wall, which, so far as I can ascertain, appears to be 

 composed of contributions from the post-frontal, parietal, malar, maxillary, and ali- 

 sphenoid bones. As in the Turtle just mentioned, the orbit likewise communicates 

 with the temporal fossa by a large aperture bounded externally by the scroll-like 

 external pterygoid process. At the back part of the inner wall of the aperture the 

 large foramen is situated, for the transmission of the trifacial nerve. 



The sutures in the fossil are, in many cases, very indistinct, and in other cases 

 the specimen has been fractured in their course, so that it is diiScult to follow them 

 or to determine the outlines of the individual bones. 



The upper surface of the parietals are flat, and exhibit no markings of investing 

 scutes, which is also the case with the other bones of the cranium. 



The frontals are small, and their upper surface is flat. They are defined by an 

 irregularly transverse suture from the parietals, by another directed forward to the 

 middle of the top of the orbits from the po^t-frontals, and by an oblique one from 

 the pre-frontals. 



The pre-frontals, as in most Turtles, are not distinct from the nasals. They are 



