OSTEOLOGY. 5 



Of the other cervicals the arches are separable from the centra along a line of 

 cartilage. The neural arches are all low and devoid of spines. There are no 

 lateral processes. As regards the articular ends of the centra, there is great diversity. 

 In Trachemys scripta, the second and third are convexo-concave; the fourth and 

 the eighth are convexo-convex; the fifth is concavo-convex; and the seventh, concavo- 

 concave. The articulations between the fifth and sixth, the sixth and the seventh, 

 and the seventh and the eighth are extended from side to side and divided medially, 

 forming a true hinge joint, which permits very free motion up and down, but 

 restricts it from side to side. The anterior and the posterior zygapophyses are 

 little elevated above the centra and are placed far apart. These arrangements 

 contribute to free motion in a perpendicular plane, but limit it in a horizontal. 



Behind the last dorsal vertebra come two sacrals. Their ribs expand distally 

 and articulate with the upper ends of the ilia. The sacrals are followed by a number 

 of caudals, about 15, but varying with the species, or even in individuals. Most 

 of these have transverse processes. 



The skull of Trachemys scripta seen from above (fig. 3) presents three pairs of 

 bones which join at the median line. In front are the prefrontals, extending back- 

 ward to the middle of the orbits. The anterior ends roof over the nasal cavity. 

 A strong process descends from each to join the vomer and the palatine, and to 

 form the front wall of the orbit. Behind the prefrontals are placed the larger 

 frontals. They aid in forming the rim of the orbits. The parietals are important 

 bones, inasmuch as they form the roof and much of the lateral walls of the brain- 

 case. The anterior end of each sends downward a strong process which joins the 

 pterygoid. Besides the pterygoid, the lower border of the parietal articulates with 

 the prootic and the supraoccipital. The latter bone is greatly prolonged backward, 

 as it is in almost all turtles. It forms a small part of the boundary of the foramen 

 magnum. 



The maxilla bounds the nasal cavity on the side, the orbit below, and its lower 

 border forms an acute cutting-edge. In life this edge is covered with a horny 

 sheath. Posteriorly the maxilla articulates with the jugal. The hinder part of the 

 rim of the orbit is formed of the jugal below and of the postfrontal above. These 

 two bones form a postorbital bar of moderate width. 



A large tympanic cavity is excavated in the quadrate, an extremely important 

 bone among the reptiles. Below the cavity mentioned, the quadrate descends to 

 form a movable articulation with the lower jaw. In the hinder border of the quad- 

 rate is a small, but deep notch for the passage of a long, rod-like bone, the columella. 

 Interposed between the anterior border of the quadrate and the jugal bone is the 

 qnadratojugal. The jugal and quadratojugal form the zygomatic arch. Above 

 and behind the quadrate is the squamosal. 



The sides of the hinder part of the skull are occupied each by a long excavation, 

 the temporal fossa. The floor of this is formed of the parietal on the inside, of the 

 prootic and paroccipital in the middle, and of the quadrate and squamosal on the 

 outside. 



Fig. 4 represents the skull of the same species as seen from below. In front are 

 the premaxilltF, bounding the nasal cavity below and entering into the roof of the 

 mouth. On each side and behind the premaxilla is the maxilla. It presents out- 

 wardly the cutting-edge already mentioned. Its inner border joins, in front, the 

 vomer, posteriorly the palatine; the middle portion is mostly a free edge, forming 

 the lateral boundary of the choana. Between the two borders is a broad triturating, 

 or alveolar, surface, which in life is covered with a horny sheath. Along the middle 

 of this, parallel with the cutting-edge, is a sharp ridge, slightly tootht. 



