102 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



tion of the skull is more serious, since the frontals are wholly gone, as 

 well as the postfrontals, the squamosals, the quadratojugals, and 

 most of the parietals and of the jugals. A study of the parts remain- 

 ing yields me the following results: 



1 . As already observed by Professor Cope, the choanae are located 

 far forward in the roof of the mouth, as they are in Chelydra; that is, 

 immediately below the union of the prefrontals with the vomer. It 

 will be seen from the figure on Plate xv that the descending processes 

 of the prefrontals have been forced by pressure into the choanae. In 

 the Cheloniidce, through the development of horizontal laminae from 

 the vomer, the maxillaries, and the palatines, the choanae are thrown 

 farther back in the roof of the mouth. 



2. A vacuity, the palatine foramen, is present on each side of 

 the roof of the mouth, lying in the point of meeting of the palatine, 

 the maxillary and the pterygoid. This foramen is present in the 

 Chelydridce and most other turtles, but is absent in the Cheloniidce and 

 the Dermochelyidce. Dr. G. Baur (Zool. Anzeiger, vol. 12, p. 45) 

 states that this foramen is placed between the palatine and the maxil- 

 lary and is present in all turtles except the two families last named. 

 However, in many, possibly all, of the Pleiirodira the foramen is 

 located in the hinder border of the palatine, out of contact with the 

 maxillary. 



3. The cutting edge of the maxillary is very narrow, as we find 

 it in Chelydra. In the Cheloniida and especially in Che Ionia the 

 alveolar wall is proportionally much deeper, as shown by the distance 

 of its lower border from the roof of the mouth and from the lower rim 

 of the orbit. 



4. The width of the horizontal alveolar surface, in proportion to 

 the width of the roof of the mouth, much more closely resembles that 

 of Chelydra than it does that of any of the Cheloniidce. The width of 

 the effective crushing surface in Toxochelys is increased by the parti- 

 cipation of the palatine. The outer edge of this bone extends from 

 the outer anterior border of the palatine foramen to a point somewhat 

 in front of the choana, nearly reaching the premaxillaries, an ar- 

 rangement whereby the maxillary is excluded from the latter opening. 

 Parallel with and close to the outer border of the palatine is a moder- 

 ate, but distinct, roughened ridge, corresponding with which there 

 was probably a rough triturating ridge on the horny beak. In Chelydra 

 and Macrochelys the palato-maxillary suture passes directly between 

 the choana and the palatine foramen; but mesiad of the suture there 

 is a slight roughened ridge, as in Toxochelys. In the structure of 

 these parts we have a combination of the characters found in both of 

 the families referred to. 



