GENERAL CHARACTERS. 905 



in the figure of the skull of Pkytosaurus among the Crocodilia. 

 Mention must also be made of the parietal foramen (fig. 824), 

 which is a vacuity occurring in the parietals of many Reptiles and 

 Amphibians. In several living forms this foramen immediately over- 

 lies an aborted median eye embedded in the subjacent tissues, and 

 totally functionless ; but it is not improbable that in the Labyrin- 

 thodonts and other early forms this eye served the purposes of 

 vision. As the attention of the palaeontologist is often directed 

 to them, we must also observe that the Eustachian tubes are canals 

 connecting the internal ear with the pharynx or gullet. Apart from 

 certain minor ossifications which will be incidentally noticed in the 

 sequel, we must also call attention to the periotic and tympanic of 

 Mammals, since among the Cetacea these bones are of great im- 

 portance to the palaeontologist. Both these bones are connected 

 with the internal ear, the first resulting from the coalescence of the 

 prootic, epiotic, and opisthotic of the lower forms, and containing 

 the cochlea of the ear ; while the latter is formed by ossification 

 in the tissues around the tympanic membrane, and also occurs in 

 some Birds. Finally, the term tympanic ring is a convenient one 

 to apply to the bones surrounding the external ear of Reptiles, and 

 especially the Chelonia. 



In regard to the mandible, or lower jaw, which we have already 

 stated to be formed by ossifications in the region of Meckel's car- 

 tilage, each half, or ramus (fig. 827), in the Sauropsida and Am- 



an co s sy 



Fig. 827. — Inner view of the left ramus of the mandible of Crocodilus. sy, Symphysis ; d, 

 Dentary ; j, Splenial ; co, Coronoid ; an, Angular ; su, Surangular ; ar, Articular. Reduced. 

 (After Cuvier.) 



phibia consists of the following five ossifications formed in mem- 

 brane — viz., dentary (d), splenial (s), coronoid (co), angular (an), and 

 surangular (su). These unite with the articular (ar), formed from 

 Meckel's cartilage, which articulates by a glenoid cavity with the 

 quadrate. In Mammals, however, there is but a single membranous 

 ossification in each ramus, which posteriorly articulates by a rounded 

 condyle with the squamosal bone of the cranium, there being appar- 

 ently no articular ossification, and no distinct quadrate in connection 

 with the mandible. 1 In the Sauropsida the quadrate articulates 



1 See the introductory chapter on the Mammalia (Chapter lvii.) 



