IN THE PARAGUAYAN LEPIDOSIREN, ETC. 365 



The connection of the pectoral girdle with the postero-lateral regions of the skull is 

 entirely by ligament, the " post-temporal " bone of Ceratodus and Protopterus being 

 wholly unrepresented. 



From what has been stated as to the structural differences and resemblances between 

 the skulls of the three genera of existing Dipnoi it may be inferred that, while all 

 three conform to the same fundamental type of structure, and at the same time exhibit 

 successive stages of modification in the same general direction, the skull of Ceratodus 

 is by far the most generalized and primitive ; and further, that of the two remaining 

 genera the skull of Lepidosiren represents but a slightly more specialized type when 

 compared with the skull of Protopterus, In support of the first statement it is only 

 necessary to recall the more complete development of the chondrocranium in Ceratodus, 

 especially in the ethmo-nasal, interorbital, and palato-pterygoid regions, the absence of 

 a fronto-parietal bone, the feeble development of the exoccipitals, the segmentation of 

 the branchial arches, the more complete hyoid arch with its hyomandibular, hypohyal, 

 and basihyal elements, the presence of a dentary splint in the lower jaw, and also 

 the retention of a series of suborbital ossicles. On the other hand, Protopterus and 

 Lepidosiren differ from Ceratodus and agree with each other in the extensive atrophy 

 of the central region of the chondrocranium, the absorption of the palato-pterygoid 

 cartilage, the characteristic constriction of the nasal from the ethmoidal region through 

 the massive growth of the palatine symphysis, the development of a fronto-parietal 

 bone, and the increased size and importance of the exoccipitals. It is none the less 

 obvious, however, that in both genera specialization due to the increased development 

 of certain cranial structures has proceeded side by side with the reduction or suppression 

 of others. Of the latter process we have examples in such modifications as the atrophy 

 of the dentary element of the lower jaw and of the suborbital bones, and the reduction 

 in the hyoid arch by the loss of its hyomandibular, hypohyal, and basihyal elements, 

 while the substitution of lungs for gills as the more important respiratory organs is 

 clearly responsible for the reduction in size and simplification of the branchial arches, 

 the closure of the hyoidean cleft, and the relatively smaller size of the opercular bones. 

 Finally, the slightly more pronounced specialization of the skull of Lepidosiren is 

 evidenced by the further atrophy of the chondrocranium and the compensating increase 

 in the size of the fronto-parietal and in the extent of its relations to the cranial and 

 suspensorial regions. To these features may be added the increased backward rotation 

 of the suspensorial cartilage, the somewhat greater development of the antorbital 

 cartilages, and the total suppression of the sixth branchial arch and of the epibranchial 

 elements of the second and third arches. Briefly, it may be said that if the skull of 

 Ceratodus be taken to represent a relatively early larval stage, the skulls of Protopterus 

 and Lepidosiren are comparable to two immediately succeeding and very much later 

 stages, while, with one or two exceptions, the differences between the two latter 

 vol. xiv. — part v. No. 6. — February, 1898. 3 B 



