338 AMERICAN TESTUDINATA. Part II. 



o 



the neck; but approaching the head there is more freedom of movement up and 

 down, and the head itself turns freely in both planes on the nearest two joints. 

 So the general direction of the bending of the neck is sidewise, and when the 

 animal resorts to the shield for protection, it turns the head to one side/ and 

 does not carry it directly back, bending the neck under the dorsal column, as 

 is the usual way. The unusual length of the dorsal spinous apophyses, and the 

 long extension of the bony walls of the body in front of the dorsal column and 

 of the first costal arch, clearly depend upon the habit of these Turtles of bending 

 the neck sidewase. The arch of the atlas is firmly fixed to its body; it is also 

 firmly fixed to the body of the epistropheus, and closes over it, so that this one 

 arch with two vertebral bodies acts fully as one vertebra, which articulates as such 

 with the occipital condyle, and the vertebra next behind. 



The head is broad across the ears, low at the hind end of the brain-box, 

 and almost flat in front of it. The middle of the floor of the skull, from the 

 occipital condyle to the alveolar surface, is almost straight. The walls of the 

 ear cavities, as they open from the Iji-ain-box, reach far forward and downward, 

 and a line across the middle of the outer ends of these cavities would pass nearly 

 over the middle of the brain-box. The brain-box is very low; the lateral 

 occipitals meet over it, and the occipital crest raises the parietals up some dis- 

 tance, but they fall fiist forward, and at their front ends the roof and floor of 

 the skull are brought together, leaving the passage from the brain cavity forward, 

 and the open space on each side, very small and low; the roof is raised a little 

 in passing forward over the cavities of the eyes and of the nose. The eyes are 

 placed far forward, and look upward as well as outward. The jugals and postfron- 

 tals are broad behind the eyes, and lie for the most part immediately upon the 

 pterygoids and palatines. There is no arch from the ear region forward, but 

 instead there is one over the temporal muscles, formed by the meeting of the 

 mastoids and parietals. The front wall of the ear cavity curves sharply forward. 

 There is a deep, large depression in the mastoid behind the os tympani for the 

 passage and attachment of the digastric muscle. The roof of the mouth is very 

 broad: the pterygoids have no depression in their outer edges; they turn down 

 on the OS tympani, reaching as low as the articulating surface, so that there the 

 roof of the mouth is a flattened arch, but at the front end it is curved up 

 toward the outer edges. 



The upper alveolar surface is merely a slight depression in the thickness of 

 the jaw. The floor of the nasal cavity projects forward beyond that surface. 



1 All the fresh-water Turtles which have this and Bibron into one group, under the name of Pleu- 

 strncture of the neck have been united by Dumeril roderes, as a sub-family of the Elodites. 



