IN THE PARAGUAYAN LEPIDOSIREN, ETC. 359 



as these structures are concerned. Protopterus, in fact, has but a single pair of upper 

 labial cartilages, which, in position and relations, are identical with those of Lepidosiren, 

 and also with the so-called " anterior labials " correctly figured by Peters sixty years 

 ago, but apparently overlooked by every subsequent writer. 



The suspensorial cartilage of Protopterus differs from that of Ceratodus in the 

 restricted antero-posterior dimension of its proximal portion, which does not therefore 

 arch backward over the branchial apparatus to the extent it does in the latter genus, and 

 also in the absence of any representative of a palato-pterygoid cartilage. The cartilage 

 figured by Wiedersheim (I. c. Taf. ii. fig. 3, tr.) fringing the dorsal border of each 

 palato-pterygoid bone, and rightly named by him the " pars trabeculars cranii," lies 

 above the level of the cranial floor, and is therefore a remnant of the lateral chondro- 

 cranial wall and not a palato-pterygoid cartilage. The suppression of the latter 

 cartilage is perhaps to be associated with the increased relative thickness of the 

 palato-pterygoid bone, and its more intimate relations with the lateral margins of the 

 basis cranii. On the other hand, Protopterus agrees with Ceratodus and differs from 

 Lepidosiren in the degree of inclination of the suspensorium to the basicranial axis. 



The two exoccipitals of Protopterus are peculiar in that they meet and fuse dorsally 

 beneath the cartilage of the supraoccipital region. Wiedersheim regards them as 

 representing a " supraoccipitale," but the resemblance of the lateral halves of the bone 

 to the paired bones of Lepidosiren, both in shape and in their relations to the foramina 

 for the exit of the Vagus and Hypoglossal nerves, is sufficiently close to justify one in 

 regarding them as a pair of confluent exoccipital elements. 



The remaining bones of the skull are on the whole very similar to those of Lepidosiren. 

 As compared with Ceratodus the most striking differences relate to the presence of a 

 fronto-parietal forming a complete gable-roof to the cranial cavity, except for a limited 

 extent between its anterior margin and the dermal ethmoid, and also, in conjunction 

 with the marginal ascending plates of the parasphenoicl, contributing to form the bony 

 side-walls of the central region of the same cavity. It may be remarked, however, that 

 in the suspensorial region the fronto-parietal is restricted to the cranial roof, and does 

 not in any way invest the outer surface of the periotic capsule or the suspensorial 

 cartilage, or hide in an external view the continuity of either with the trabecular 

 cartilage. To these features may be added the absence of a complete supracranial 

 roof, due to the want of a " scleroparietal" and of the superficial lamellar plates of 

 the squamosal, combined with the relative narrowness of the dermal ectethmoids. The 

 characteristic preorbital or nasal section of the dermal ectethmoid of Ceratodus is 

 wholly wanting in Protopterus, the bone being displaced upward and backward by 

 the growth of an exceptionally large ascending process from the palato-pterygoid bone. 

 Nevertheless, the dermal ectethmoids are much better developed than in Lepidosiren, 

 and, in the absence of a " scleroparietal," their expanded anterior portions meet in a 

 median suture dorsad to the fronto-parietal. 



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