364 PEOF. T. W. BRIDGE ON THE MORPHOLOGY OP THE SKULL 



The fronto-pavietal is a relatively much larger bone in every dimension than in 

 Protopterus. In the anterior part of its extent not only does the bone completely 

 invest the dorsal surface of the cranium, but through the extension of its lateral or 

 descending plates as far forward as the ascending processes of the palato-pterygoids, 

 and posteriorly as far back as the periotic capsules, the lateral walls of the entire pre- 

 auditory section of the cranial cavity are formed wholly by bone. Hence it follows 

 that the lateral cranial vacuities of Protopterus have no existence in Lepidosiren, 

 or at most are only represented on each side by a narrow suture, or rather fissure, 

 between the ascending process of the palato-pterygoid and the anterior and lateral 

 margins of the fronto-parietal, while the more or less extensive area of trabecular 

 cartilao-e which, in a vertical longitudinal section of the skull of Protopterus, is visible 

 in front of the periotic capsule, becomes entirely concealed from view in Lepidosiren. 

 More posteriorly, in the suspensorial region, the fronto-parietal of Protopterus is 

 confined to the cranial roof, and, except for its partial investment by the squamosal, 

 the whole extent of the suspensorial cartilage and its continuity with the periotic 

 capsule and the trabecular cartilage are clearly visible in an external and lateral view. 

 In Lepidosiren, on the contrary, a lateral and downward extension of the bone almost 

 completely invests the outer surfaces of the periotic and suspensorial cartilages, and, in 

 a similar view, effectually hides the continuity of the latter with the trabecular 

 cartilage. The sagittal crest is somewhat better developed in Lepidosiren than in 

 Protopterus, and to this may be added a " lambdoid " ridge, which, owing to the 

 relative narrowness of the hinder portion of the fronto-parietal, is scarcely indicated in 

 the latter Dipnoid. 



The dermal ectethmoids are scarcely so well developed as in Protopterus. Their 

 anterior portions are much narrower and fail to meet in a median suture : hence the 

 fronto-parietal and its characteristic crest may be seen between them in a dorsal view 

 of the skull. (Compare PI. XXVIII. fig. 2, and Wiedersheim, I. c. Taf. ii. fig. 1.) 



Lepidosiren has but five simple, unsegmented, branchial arches, the sixth arch, like 

 the fifth cleft, having been suppressed i. As compared with Protopterus, a noticeable 

 feature is the exceptional thickness of the second arch, and its dorsal cleft or groove 

 for an aortic arch. 



As previously mentioned, the position and relations of the foramina for the exit of 

 the cranial nerves are nearly identical in the two genera. It may be pointed out that 

 the Optic nerves in Lepidosiren escape through the two clefts or fissures which 

 represent the anterior cranial vacuities of Protopterus, and furtlier that in the former 

 Dipnoid only the roots of the first spinal nerve (Hypoglossal) perforate the exoccipital, 

 the second pair simply traversing the fibrous tissue which intervenes between that bone 

 and the first neural arch. 



' The distinct epibranchial elements of the second and third arches in Protopterus are wholly unrepresented 

 in Lepidosiren 



