60 MR W. E. AGAR ON THE DEVELOPMENT OF THE 



Furbringer rejected the supposition that they are lower labials on the ground that 

 they have no representatives in Ceratodus, and suggests the name paramandibulars 

 for them. 



The fronto-parietal bone at this stage (fig. 16) shows a distinctly intermediate form 

 between that of the adults of Lepidosiren and Protopterus. A very large proportion 

 of the auditory capsule is left exposed, owing to the slight ventral extension of the 

 bone, while anteriorly the lateral plate, i.e. that part of the bone which forms the side 

 wall of the cranium above the trabecula, leaves a considerable fenestra, closed by 

 membrane, between it and the ascending process of the palato-pterygoid. The optic 

 nerve leaves the skull through this foramen. 



It will be noticed that the foramen for the oculo-motor is placed in a slightly 

 different position to that figured by Bridge. The course of this nerve through the 

 orbito-temporal process, and its application to the ophthalmicus profundus branch of 

 the trigeminal, has already been mentioned ; the two nerves run close together in a 

 groove between the cartilage and the inner surface of the bone, emerging together under 

 the edge of the latter as shown. 



As regards the development of the patch of cartilage on the anterior surface of the 

 distal end of the hyoid, supposed by Bridge to represent a vestigial hyoidean ray, there 

 is not much to say. It appears very late, not being present even at stage 38 (about 

 three months after hatching). I found it in sections of a Lepidosiren of about eighteen 

 months, and also in a Protopterus of 7*5 cm. It lies, as Bridge describes, external to 

 the osseous sheath. 



Lack of the necessary stages has prevented me from going into the question of the 

 patch of cartilage found on the anterior surface of the occipital rib in Protopterus, and 

 to which various homologies have been ascribed. It does not appear till after stage 36. 



In the change from stage 38 to the adult, there is no absorption or replacement of 

 cartilage, except in the pleuro-occipital region in Lepidosiren. The sheath of bone 

 round each occipital arch increases in thickness, and finally forms a solid bone with a 

 deep notch in its dorsal end into which the supra-occipital cartilage extends.* The bone, 

 at first circular in section like the other neural arches, becomes pulled out in an antero- 

 posterior direction, remaining constricted in the middle, however, by the notches for the 

 exit of the occipital nerves in front, and of the spino-occipital a behind. 



Otherwise the chondrocranium of the adult is in both genera more complete than 

 in the young form. In Lepidosiren, in the auditory region the cartilage spreads 

 further up under the fronto-parietal, without, however, meeting in the middle line. 

 The shelf of cartilage under the squamosal increases in width and thickness. The 

 backwardly projecting styliform process of the mesethmoid cartilage is not, as Bridge 

 suggests, a remainder of the more extensive cartilaginous cranial roof of the young. 

 The mesethmoid itself first appears in stage 36 -f as a backward extension of the 

 internasal septum (cf. figs. 14 and 16), and thence increases to the adult size. 



* The cartilage i.s not absorbed, but squeezed out at each end of the bony sheath as this thickens. 



