164 C Judson Herrick 



Summary. — The elasmobranch forebrain is specialized in a peculiar 

 and aberrant direction. The olfactory bulbs are widely evaginated and 

 the remainder of the telencephalon is characterized by thickening, inver- 

 sión, and partial evagination at the rostral end, while the caudal end 

 remains in a very undifferentiated condition as telencephalon médium. 

 The área olfactoria makes up the enlarged lateral lobes and its dorsal 

 subdivisión, which is primordial archipallium, has advanced further toward 

 the cortical type of higher vertebrates than in the cartilaginous ganoid 

 fishes, though its morphological relations are peculiar. The narrow 

 telencephalon médium is occupied largely by the área somática, a non- 

 olfactory región in intímate functional relations with the somatic sensory 

 and motor centers of the thalamus and with the área olfactoria lateralis. 

 The relations of the dorsal, ventral and intermedíate zones of the forebrain 

 of Acanthias are shown in figure 18, and they may be tabulated as foUows: 



Telencephalon 



üiencephalon 



Área olfactoria dorsalis, stria medullaris Habenula. 



)lfactoria laterahs, área somática : Thalamus. 



1 Área ol 



yi ,, ,r I Área ol 



biilbus olí.' T- u 1 1Í- i • 



I 1 uberculum oliactonum 



f Área olfactoria medialis, nucleus preopticus Hypothalamus. 



Teleostomi. 



The Ganoidei and Teleostei present a graded series from very primi- 

 tive cartilaginous types like Acipenser to the very highly specialized bony 

 fishes. The bony ganoid. Amia calva, occupies an intermedíate position 

 in this series and is chosen as typical of this heterogeneous group. The : 

 forms of forebrain in the Teleostei are exceedingly diversified but all of fl 

 these forms have been derived from the primitive ganoidean type of Aci- 

 penser by modifications of a single general tendency, viz., the great 

 thickening of the lateral w^alls of the telencephalon médium and the lateral 

 eversión of their dorsal borders with consequent widening of the mem- 

 branous roof (figs. 19 to 23). 



In all Teleostomi the bulbus olfactorius is typicaliy evaginated and in 

 some species the crus olfactorius is greatly elongated; the remainder of the 

 telencephalon is unevaginated and more or less everted. There is no 



