6 FOSSIL TURTLES OF NORTH AMERICA. 
The midline behind the premaxillz 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 bastocci p1- 
tal. It is joined on each side by the exocci pital, 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 1s the dentary, 
furnishing the triturating surface of the jaw, covered above with a horny sheath, and 
WwW 
Figs. 3 and 4. Trachemys scripta. 
3- Skull seen from above. X1. fr, frontal; ju, jugal; pa, parietal; paoc, paroccipital; pfr, prefrontal; pof, post- 
frontal; pro, prootic; gu, quadrate; sg, squamosal; soc, supraoccipital. 
4. Skull seen from below. 1. a/v, alveolar surface of maxilla; boc, basioccipital; bsp, basisphenoid; exc, exoc- 
cipital; mx, maxilla; pal, palatine; paoc, paroccipital; pmx, premaxilla; pro, prootic; pt, pterygoid; qu. art, 
articulation of quadrate with lower jaw; gj, quadratojugal; sg, 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 coronord 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, x1, 1896, p. 413) calls the lower of 
the two the splenzal, the upper the angular.’ Williston (Science, XVIII, 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, 
