82 COMPARATIVE ANATOMY 



that of Amphibia on the other. In certain respects, however, it 

 presents special characters in which it differs from that of all 

 these forms. 



The chondrocranium is retained either entirely (Ceratodus) or 

 at any rate to a large extent (Protopterus and Lepidosiren), and 

 the cartilage bones are much less numerous than in Ganoids, 

 exoccipitals only being present (Fig. 62). The cranial cavity 

 extends forwards between the orbits to the ethmoidal region, and 

 the lamina cribrosa is largely cartilaginous. The quadrate, which 

 is covered by a squamosal (which corresponds to the preopercu- 

 lum of Fishes), is fused with the cranium, and the connection 

 between the latter and the strongly ossified palatopterygoid, which 

 unites with its fellow anteriorly, is a very close one. 



The lattice-like cartilaginous nasal capsules are situated right 

 and left of the apex of the snout, close under the skin. As in 

 all the higher Vertebrates, each nasal cavity communicates with 

 the mouth by internal nostrils (choance) as well as with the exterior 

 by the external nostrils, which are, however, covered by the upper 

 lip. The labial cartilages are directly connected with the inter- 

 nasal septum. 



The occipital region is immovably connected with the 

 vertebral column, some of the anterior vertebrae being firmly 

 united with the skull. The teeth, which are sharp and blade- 

 like, are covered with enamel, and are borne on the palatoptery- 

 goid and mandible ; small " vomerine teeth " are also present, 

 though there is no vomer. The gill-covers and branchiostegal 

 rays are greatly reduced, and even the five cartilaginous gill- 

 arches are in a very rudimentary condition in Protopterus and 

 Lepidosiren. 



The strong lower jaw is ossified by an articular, a dentary, an 

 angular, and a splenial, on the last mentioned of which the teeth 

 are borne ; Meckel's cartilage extends for a short distance an- 

 teriorly to the dentary. 



The Dipnoi are an extremely ancient group ; they existed in the Trias and 

 Carboniferous periods, and possibly even extended into the Silurian. Several 

 facts as regards their skull cannot be satisfactorily elucidated until something is 

 known of its development. The morphology of the so-called "cranial rib " 

 (Fig. 62), for instance, is not at present understood. 



c. Amphibia. 



Urodela. The comparatively simple skull of tailed Amphi- 

 bians is distinguished from that of bony Fishes in general 

 principally by negative characters, on the one hand by the 

 presence of less cartilage in the adult, and on the other by 

 a reduction in the number of bones. In the larval condition 

 (Fig. 63), the chondrocranium, with its nasal, orbital, and auditory 



