130 ANATOMY OF VEItTEBKATES. 



canal, the bone so called being ossified continuously, as a process, 

 from the maxillary. The premaxillaries, figs. 90, 91, 22, closing the 

 arch anteriorly, are very small in all Chelonia, and the sutures 

 marking them oft' from the mamillaries are wanting in some 

 Mud-turtles (Tetronyx lonyicollis, Fitz. Trionyx Bibroni): 1 the 

 premaxillary part of the facial profile is vertical in many Chelonia, 

 as in fig. 91 : but in Tetronyx it extends from the nostril down- 

 ward and backward — the reverse of prognathism. 



The pterygoid, fig. 90, -/.a, diverges from the vomer and pala- 

 tine, or from the palatine and maxillary, fig. 98 B, backward and 

 outward: uniting, in Chelone, with its fellow below the basi- 

 sphenoid, fig. 90, 5, and diverging outward and backward to abut, 

 at a, against the tympanic, 28. In some Soft-turtles, e.g., Trionyx 

 ( Gymnojms) indiens, the vomer is directly continued from the basi- 

 pre-sphenoid, and divides the pterygoids from each other. 



A second outer bar of bone, fixing the maxillary arch to the 

 tympanic, is present in all Chelonia, and divided into two pieces. 

 The proximal piece, fig. 91, 26, is articulated with the maxillary, 

 21, enters into the lower and back part of the orbital border, unites 

 superiorly with the postfrontal, 12, and posteriorly with the second 

 piece, 27. To the bone, 26, the term ' malar ' is given ; to the 

 bone, 27, the name ' squamosal.' The latter, resembling a vertical 

 scale or plate, articulates above with the postfrontal, 12, and 

 mastoid, 8 ; and behind with the tympanic, 28. It completes the 

 arch called ' zygomatic,' bounding externally the temporal fossa, 

 which is roofed over by bone in the Turtles (figs. 89 and 91), and 

 a few Emyds ; but is widely open above in other Chelonia. 



The tympanic pedicle is a single bone, fig. 91, 28, expanded above, 

 with a more or less circular border for the insertion of the mem- 

 brana tympani ; excavated internally by the tympanic air-cells ; 

 notched behind for the reception of the columellar stapes, as in 

 the Turtles, fig. 91 ; with a narrower cleft in Tetronyx, and with 

 the borders uniting in the Tortoises and some other Chelonia, 

 reducing the stapedial passage to a foramen or canal, fig. 92, 28. 

 The lower end of the tympanic supports a transversely extended 

 condyle with an undulated or nearly flattened surface. The tym- 

 panic articulates above with the paroccipital, fig. 89, 4, in some 

 species with the alisphenoid, in others with the superoccipital, as 

 well as with the mastoid, ib. and fig. 91, 8. 



The mandible consists of an 'articular' element, small, but dis- 

 tinct in the Turtle, fig. 91, so; connate in Emys with the ' suran- 



1 xliv. No. 954, p. 185. 



