ELASMOBRANCH BRAIN ORGANIZATION 175 



(Ebbesson and Heimer 1970, Bruckmoser and Dieringer 1973, Ebbesson and 

 Northcutt 1976). 



In sharks, two main telencephalic areas receive ipsilateral secondary olfac- 

 tory fibers: the lateral pallium (lateral olfactory area of Ebbesson and 

 Heimer, 1970), and a lateral portion of area superficialis basalis (Figures 6, 

 10). In Raja eglanteria, the secondary olfactory projections (Northcutt and 

 Boord, unpublished observations) are confined to the lateral hemispheric 

 wall, terminating in the lateral pallium (lp, Figure 15B, C, D) and in a more 

 medial nucleus termed nucleus a (a, Figure 15B). 



Secondary olfactory fibers do not extend to large portions of the tel- 

 encephalon in sharks and skates. However, it could be argued that the tel- 

 encephalon is concerned mainly with olfactory information if the centers 

 receiving direct olfactory input projected widely within the telencephalon. 

 Ebbesson (1972), though, has shown that the lateral olfactory area in 

 Ginglymostoma reveals only limited telencephalic projections. It does pro- 

 ject to the ipsilateral area superficialis basalis and, quite sparsely, to the 

 contralateral lateral olfactory area and area superficialis basalis, thus avoid- 

 ing the bulk of the telencephalon. 



At present, it is difficult to compare the overall pattern of elasmobranch 

 olfactory organization to that of other vertebrates. Sharks and skates are 

 similar to mammals in that the secondary olfactory projections are ipsilateral 

 and terminate in only a portion of the ipsilateral hemispheric wall; olfactory 

 projections are bilateral in all other anamniotes. This difference is par- 

 ticularly striking when elasmobranchs are compared to actinopterygian 

 fishes, which reveal bilateral olfactory projections to the telencephalon, as 

 well as direct projections to the hypothalamus (Finger 1975, Northcutt and 

 Braford 1977). 



However, the main olfactory target of elasmobranchs, unlike that of mam- 

 mals, does not project outside the telencephalon but to other telencephalic 

 areas, as in amphibians and reptiles. Thus, the overall pattern of olfactory 

 organization in elasmobranchs is unique among vertebrates; yet certain 

 features of that pattern characterize many other vertebrate groups. 



There are no experimental data for olfactory projections in chimaerids. In 

 fact, earlier descriptive studies do not agree about the extent of the second- 

 ary olfactory fibers or their probable targets. Holmgren (1922) and Kuhlen- 

 beck and Niimi (1969) established the pallio-subpallial boundary high on the 

 lateral telencephalic wall (position 1 in Figure 28); Faucette (1969a, 19696) 

 established a boundary much lower on the telencephalic wall (position 2 in 

 Figure 28). Thus, two different cell groups in the chimaerid telencephalon 

 have been interpreted as the homolog of lateral (piriform) pallium. Holmgren 

 (1922) and Kuhlenbeck and Niimi (1969) argued that the cell group imme- 

 diately dorsal to position 1 was the lateral pallium. Faucette (1969a, 

 1969b) argued that a more ventral cell group (cell group w in Figure 28) 

 was the lateral pallium. In Hydrolagus, a compact, heavily myelinated 

 bundle can be traced from the lateral surface of the olfactory bulb, and must 

 represent the main, if not sole, secondary olfactory pathway (ot, Figure 28). 

 My own analysis of the telencephalon of Hydrolagus suggests, as did 



