C'HFXONIA 



wide notch behind ; in the Pleurodira this part of the quadrate 

 is transformed into a trumpet, tlie wide rim of which, forming a 

 complete ring, carries the tympanic membrane. The tympanic 

 cavity thus formed often leads into a deep recess which extends 

 beneath the squamosal towards the opisthotic and bears some 

 resemljlance to the intricate tympanic recesses which pervade 

 that region of the Crocodilian skull. 



Dorsally the quadrate is broadly overlaid by the squamosal, 

 which frequently forms an arch with the parietal. Anteriorly 

 the quadrate is connected through a variably sized quadrato- 

 jugal with the jugal ; and this, by joining the maxilla and post- 

 frontal, helps normally to form the posterior rim of the orbit. 

 All the bones which border the temporal fossa vary much in 

 extent in the different groups of Chelonia. The extremes are 

 represented by Cistudo and Geoemyda, in which the liony infra- 

 temporal arch is absent, owing to the loss of the quadrato-jugal ; 

 and on the otlier hand by the Chelonidae and by Siihargis, in 

 which the whole temporal region is covered over by an additional 

 " false cranial " roof This roof is produced chiefly by lateral 

 wing -like expansions of the parietal and postfrontal bones, 

 which meet the likewise much expanded jugal, quadrato-jugal, 

 and squamosal bones. In the lower diagram of Fig. 63 {Chelone 

 mydas) the squamosal has been removed, and the other bones 

 have been reduced to their normal, or rather primitive condition, 

 for comparison with the external view of the complete skull of 

 the same animal. The lower diagram shows also the connexion 

 of the pterygoid with a descending process of the parietal ; this 

 column, paired of course, usually contains a separate bone, the 

 epipterygoid, the portion between Ptg and Par. 



The hyoidean apparatus is well developed, and sometimes 

 assumes large dimensions, especially in Chclys. The two pairs 

 of " horns " are the first and second branchial arches, whilst the 

 hyoid arches are reduced to a pair of small, frequently only 

 cartilaginous, nodules attached near the anterior corners of the 

 basis linguae, which generally fuses with the os entoglossum in 

 the tip of the tongue. 



The pectoral arch consists of a pair of long coracoids sloping 

 obliquely backwards, the distal cartilages of which scarcely 

 touch each other in the middle line, and the scapulae. The 

 upper end of the scapula frequently touches the inside of the 



