26 



PROF. T. H. HUXLEY ON CERATODXJS FORSTEKI. [Jan. 4, 



An arrangement of a very similar character exists in Chimcera. 

 Here the nasal septum is very narrow, but widens out below, where, 

 as the prsemaxillary lip, it overlaps the vomerine teeth. The free 

 edge of the septal ala is curved in, as in Cestracion. Meeting it is 

 an inward process of the maxillary lip, which abuts against the septal 

 ala in the same way as the maxillary ala does in Cestracion. (Out- 

 side this, again, is another flap-like process of the maxillary lip, 

 which overhangs the foregoing when the maxillary lip is in place. 

 Between the prsemaxillary lip and the maxillary lip is the nasal pas- 

 sage, open ventrally as in Cestracion ; and an interval between the 

 vomerine and palatine teeth above and the mandibular tooth below 

 (the posterior nostril) places this passage in free communication with 

 the oral cavity. 



It is obvious that if the septal and the maxillary alae in Scyllium, 



Fig. 1. 



TV 



Ceratodus forsteri. Dorsal view of the brain in situ. 

 Pr E, lobus communis of the prosencephalon; Th.E, Thalamencephalon ; Pn, 

 pineal gland ; ME, mesencephalon (the median groove is somewhat too 

 strongly marked) ; Cb, cerebellum ; Tv, tela vasculosa over the fourth ven- 

 tricle ; e.s.c, p.s.c, external and posterior semicircular canals ; I, V, VII, 

 IX, X, cerebral nerves ; E. 0, esoccipital ossification. The general contour 

 of the chondrocranium is given ; on the right side the cartilage has been 

 sufficiently removed to show the anastomosis of the seventh and ninth 

 nerves, the auditory organ, and other deep-seated parts. With respect to 

 this and the other figures, I may remark that my object has been to make 

 accurate diagrams drawn to scale, and not pictures. 



Cestracion, and Chimcera united along the middle line, such a nasal 

 passage as exists in Ceratodus would be the result. Compared with 



