6 FOSSIL TURTLES OF NORTH AMERICA. 



The midline behind the premaxillae is occupied by the single vomer. Anteriorly 

 it divides the nasal passages from each other; laterally it articulates with the 

 palatines; posteriorly, with the pterygoids. The palatines assist in roofing the 

 nasal passages and in forming the triturating surface mentioned. Between each 

 and the maxilla of its side is seen an opening, the posterior palatine foramen. 



The pterygoids meet each other at the midline anteriorly, but posteriorly are 

 separated by the basisphenoid. They extend backward so far as to exclude the 

 bone last mentioned from contact with the quadrates. The lateral border of each 

 pterygoid is mostly a sharp free edge. Behind the basisphenoid comes the basiocctpi- 

 tal. It is joined on each side by the exoccipital, and all three of these bones join in 

 forming the occipital condyle. From this view is seen also a portion of the par- 

 occipital and squamosal. On each side of the basicranial axis are seen foramina 

 for the passage of various nerves and blood-vessels. 



Each ramus of the lower jaw is composed of six bones. In front is the dentary, 

 furnishing the triturating surface of the jaw, covered above with a horny sheath, and 



Pfr 



pmx 



Figs. 3 andj4. Trachemys scripts. 



3. Skull seen from above. Xi. jr, frontal; ju, jugal; fa, parietal; paoc, paroccipital; pfr, prefrontal; pof, post- 



frontal; pro, prootic; qu, quadrate; sq, squamosal; soc, supraoccipital. 



4. Skull seen from below. Xi. alv, alveolar surface of maxilla; boc, basioccipital; bsp, basisphenoid; exoc, exoc- 



cipital; mx, maxilla; pal, palatine; paoc, paroccipital; pmx, premaxilla; pro, prootic; pt, pterygoid; qu. art, 

 articulation of quadrate with lower jaw; qj, quadratojugal; sq, squamosal; vom, vomer. 



completely co-ossified with its fellow of the opposite side at the symphysis. On the 

 lower border of the jaw this bone extends backward nearly to the articulation with 

 the quadrate. The upper border of the jaw, behind the triturating surface, is formed 

 in front by the coronoid bone; posteriorly by the supraangular. These two bones 

 are to be seen both from the outside and from the inside of the jaw. Behind the 

 supraangular is a nodular bone that articulates with the quadrate, the articular. 

 On the inner surface of the jaw, near the hinder end, are two bones, whose names 

 are in dispute. Baur (Anatomischer Anzeiger, xi, 1896, p. 413) calls the lower of 

 the two the splenial, the upper the angular. Williston (Science, xvni, 1903, p. 830) 

 regards the lower bone as the angular, the upper as a dermal articular. The same 

 author has, in his work on North American Plesiosaurs, 1903, page 30, called the 

 latter bone the prearticular, and this name is adopted by the present writer. 



The shoulder-girdle of an emyd turtle consists of two bones on each side. One 

 of these, the scapula, consists of two slender portions placed at nearly a right angle 

 with each other. The longest portion, the proper scapula, the body of the scapula, 



