SKULL AND VISCERAL ARCHES IN LEPIDOSIREN AND PROTOPTERUS. 61 



In a Protopterus of 7*5 cm. long the auditory capsules are connected dorsally from 

 about the middle of their length backward. The anterior part of the roof is separated 

 from the supra-occipital cartilage by a region of much thinner cartilage, probably 

 indicating the formation of a tectum synoticum distinct from the supra-occipital 

 cartilage. The spine arising from the anterior end of the Gasserian recess in Protopterus 

 of about stage 36 (fig. 15) is the forerunner of a great increase in the dorsal extent of the 

 wall of the recess. In the 7 '5 cm. specimen the space between this and the auditory 

 capsule has become filled up with cartilage. 



Another example of the increase in the bulk of cartilage which takes place between 

 the stage 36 and the 7 '5 cm. Protopterus is the fact that whereas in the former, as in 

 all stages of Lepidosiren, the external carotid enters the skull by the hyoideo-mandibular 

 foramen and runs over the floor of the Gasserian recess to the foramen pro-oticum 

 where it issues to the exterior, in the latter the artery is embedded in the floor of the 

 recess, instead of coursing freely through its cavity. 



Thus it appears that ontogeny gives no support to the prevailing view that the 

 complete chondrocranium of Ceratodus exhibits a more primitive and ancestral con- 

 dition than that of the Dipneumona, and that of the latter, Protopterus retains its skull 

 in a more primitive condition than Lepidosiren* In many respects the adult 

 Lepidosiren is in the same condition as a larval Protopterus. For instance, the 

 following points are common to both : the absence of a tectum synoticum, the small 

 extent of the mesethmoid cartilage and its styloid process (these are of much greater 

 extent in the adult Protopterus than in Lepidosiren), the course of the external 

 carotid through the cavity of the cranium between the hyoideo-mandibular and pro-otic 

 foramina. Also in the very young Protopterus the foramen pro-oticum is undivided, 

 but it must be noted that the division takes place at an early stage. None of these 

 apparently more primitive characters are, so far as we can learn from ODtogeny, 

 coenogenetic. On the other hand, as regards the visceral arches and the neural arch 

 a-b, Ceratodus is in the most primitive condition. 



Summary. 



The extreme anterior end of the notochord degenerates, and is replaced by forward 

 growth of the definitive notochord. 



The trabeculse are the first parts of the skull basis to appear. 



The " Balkenplatten " appear subsequently to the trabeculse, but in continuity with 

 them. There is no distinct mesotic cartilage. 



The occipital arch has the form of a neural arch. The occipital plates grow forward 



* It should, however, be mentioned that Wiedersheim (loc. cit.) found in a young Protopterus that the meseth- 

 moid filled up the whole space between the ascending processes of the palato-pterygoids, i.e. was more extensive than 

 in the adult. In my 7'5 cm. specimen the mesethmoid had the adult form of a narrow, backwardly projecting spine 

 of the internasal septum. 



