1887.] PROF. T. J. PARKER ON CARCHARODON RONDELETII. 37 



rest.) are large and folded. The metacoele or fourth ventricle (mt. 

 cce.) is covered by a thick tela vasculosa {tel.vasc.) and its floor is 

 marked by five well-marked grooves. 



The epencephalon or cerebellum (e^oew.) is large, divided into lobes 

 bv transverse sulci, and prolonged forwards so as to cover all the 

 median portion of the mesencephalon as well as backwards over the 

 anterior sixth of the metencephalon. It contains a large epicoele 

 (cerebellar ventricle), which communicates by a comparatively small 

 aperture or ostium with the metacoele. 



The mesencephalon presents dorsally a pair of large optic lobes, or 

 optencephala (opten.), and contains a spacious mesoccele (fig. 21, 

 {mes.cce.), on the middle cf the floor of which is a small aperture 

 (.r) leading into the diaccele or third ventricle (di.coe.). This 

 apparently unusual arrangement no doubt indicates that the optic 

 lobes have overlapped the posterior portion of the diencephalon. 



The diencephalon {dien.)\s short; its cavity, the diaccele (rfz.cce.), 

 is arched over posteriorly by a narrow bridge of nervous matter, but 

 for the most part is covered in only by the thick vascular velum 

 interpositum {yel.int.). On its floor is a longitudinal groove (y) 

 leading both into the mesoccele and into the infundibulum, which is 

 short and bears a large trilobed hypophysis or pituitary body (Jiijp.). 

 No hgematosac (saccus vasculosus) was apparent, and the conarium 

 or pineal body was not observed. On the ventral surface of the 

 diencephalon are small rounded lobi inferiores {lob.inf.). 



In the fcetal brain, as well as in that of Lamna, the diencephalon 

 is quite concealed in a view from above, the anterior faces of the 

 oiJtic lobes being quite vertical and in close contact with the posterior 

 face of the cerebrum (fig. 29). 



The cerebrum is a large, transversely elongated mass, consisting of 

 the fused prosencephala or cerebral hemispheres (prosen.), the line 

 of junction between which is marked both above and below by a 

 distinct groove. Each prosencephalon is also divisible into a large 

 dorso-lateral and a smaller ovoidal ventral lobe (fig. 22). 



The cerebrum contains well-developed lateral ventricles or proso- 

 coeles (fig. 21, prs.cce.), communicating each by a foramen of Monro 

 (for.M.) with a small triangular space, the aula, lying immediately 

 cephalad of the diaccele proper, and consisting of the cavity of the 

 basi-cerebrum or unpaired portion of the protencephalon (embryonic 

 fore-brain) left by the budding-off of the cerebral hemispheres. 



A choroid plexus {cli.plx.) is continued into each prosoccele from 

 the velum interpositum. On the inner wall of the cavity is a large 

 ovoidal elevation (a), and a smaller one (6) occurs on its floor. 



The rhineucephalon consists, as already stated, of a greatly 

 elongated cms, and of a bulb in apposition with the olfactory sac. 

 A cavity, the rhinocoele {rh.cce.) is continued into the crus from the 

 corresponding lateral ventricle. 



The first four cerebral nerves present no special features ot 

 importance. The fifth, seventh, and eighth arise, as usual, close 

 together, having between them four principal roots. 



The trigeminal (figs. 20, 22, and 23, v) arises by two roots— an 



