IN THE PARAGUAYAN LEPIDO SIREN, ETC. 351 



passages, situated immediately dorsad to the palato-pterygoid symphysis and provided 

 with complete cartilaginous walls, continuous behind with the rest of the chondro- 

 cranium. Mesially, the two passages are separated by a thick vertical mesethmoid 

 cartilage (PI. XXIX. fig. 20, ms.e.). More anteriorly the floor and outer wall of each 

 olfactory passage terminate somewhat abruptly, leaving a large foramen for the entrance 

 of an Olfactory nerve into the nasal sac of its side, but the mesethmoid cartilage and the 

 overlying chondrocranial roof are directly continuous with the internasal septum and 

 with the cartilaginous laminae which grow out on each side from the dorsal edge of 

 the septum to form the irregularly-fenestrated roof of the nasal capsules. The more 

 normal condition of the mesethmoid region in Ceratodus may be associated with the 

 relative thinness of the symphysial portions of the two palato-pterygoid bones, so that 

 the vertical constriction of this region by the excessive thickening of the symphysis, 

 which is so characteristic a feature in Lepidosiren, does not take place, and, in con- 

 sequence, the mesethmoid cartilage and internasal septum are freely continuous, and 

 the palatine symphysis remains widely separated from the chondrocranial roof. Com- 

 parison of figs. 12 and 20 (PL XXIX.), representing equivalent sections through the 

 symphysis in Ceratodus and Lepidosiren, will sufficiently illustrate these remarks. 



The roof of the two nasal sacs terminates somewhat in front of the internasal septum 

 in a thin, marginally-rounded lamina of cartilage (Huxley, /. c. fig. 7, a.), without 

 presenting any recognizable indications of trabecular cornua, and without exhibiting so 

 much as a vestige of the bicornuate prenasal process of other Dipnoi. 



The presence of representatives of the antorbital cartilages of other Dipnoi is 

 doubtful. The central trabecular region of the chondrocranium is directly continuous 

 with the cartilaginous side-walls of the mesethmoid region, internally to the junction 

 of the preorbital portion of the dermal ectethmoid and the ascending process of the 

 palato-pterygoid ; and if, as Pose [36] suggests, the hinder upper labial cartilages of 

 Huxley are really the equivalents of antorbital processes, it is obvious that they have 

 lost their primitive continuity posteriorly with the trabecular cartilage (PL XXIX. 

 fig. 20, an.]).). 



The characteristic plug of dense fibrous tissue which in Lepidosiren fills up the 

 anterior section of the cranial cavity is entirely absent in Ceratodus, and the anterior 

 extremity of the brain is in close relation with the hinder edge of the mesethmoid 

 cartilage. 



Posteriorly, the separation between the skull and the vertebral column is less evident 

 than in either Lepidosiren or Protqpterus, inasmuch as at least two pairs of " basi- 

 dorsals," representing the lateral elements of two neural arches, are partially confluent 

 with one another and also with the cartilage of the exoccipital region of the skull. 



The suspensorium approximates more closely to the typical autostylic or Chimaeroid 

 condition than in any other existing Dipnoid. The metapterygo-quadrate cartilage is 

 very strongly developed, especially behind, where it is prolonged into a thin but wide 



2z2 



