232 



THE ELASMOBRANCH FISHES 



although in it fusion is greater. From Heterodontus to Scoliodon (fig. 211a) 

 a series of forms may be obtained, showing a progressive specialization to a 

 condition in which practically no sign of right and left lobes remains. 



In the component parts of the telencephalon there is also great diversity. 

 The median olfactory nucleus (m.n.) produces very different effects depend- 

 ing on the degree of its own development and also on the degree of fusion of 

 the right and left telencephalic lobes. In Scymnus these nuclei constitute a 



Fig. 211. Dorsal view of brain. (From Locy.) A. Scoliodon. B. Haja. 



cb., cerebellum ; c.r., restif orm body ; di., diencephalon ; m.n., median olfactory nucleus ; 

 med., medulla; np., neuropore ; ol.b., olfactory bulb; oJ.l., olfactory lobe; ol.t., olfactory 

 tract; op.L, optic lobe; tJ., telencephalon; /«., terminal nerve; I, olfactory nerve; II, optic 

 nerve ; IV, trochlearis or fourth nerve ; X, vagus nerve. 



pair of rounded masses located terminally between the olfactory tracts (m.n., 

 fig. 213) and extending ventrally. Much the same condition obtains in Squalus 

 sucklii (see p. 183, fig. 172a), but in this form the median nucleus is not pro- 

 jected forward into so sharp a mass. In a type like Heterodontus it stands out 

 in a particularly striking way. In many otlier forms, on the contrary, the 

 median olfactory nucleus is poorly developed. 



The pallial eminences also are very differently developed in different Elas- 

 mobranclis. In Scynuiu.^ again they are sharply defined as prominent dorsal 

 mounds l)uilt up of masses of cells (p.e., fig. 213a). In other forms these lobes 

 are less well developed (Acanthias) ; while in still others they are scarcely 

 discernible. 



The diencephalon (di.) in general maj^ be said to be narrow from side to side 

 and show no marked swellings. In some forms it is well seen from above, as in 

 Heterodontus (fig. 210) and Raja (fig. 211b) . In all such occurrences the brain 

 stem is well drawn out in this region. In those Elasmobranchs in which a 



