128 



VISION 



lmm 



Figure 3 Transverse sections through (A) rostral and (B) midmesencephalic levels of 

 Squalus acanthias. BON, basal optic nucleus; C, central tectal zone; CE, corpus of cere- 

 bellum; HY, hypothalamus; IC, intercollicular nucleus; IL, inferior lobe of hypothalamus; 

 IN, nucleus interstitialis; B°, interpeduncular nucleus; MLF, medial longitudinal fascic- 

 ulus; NP, nucleus profundus mesencephali; T, tegmentum; TS, torus semicircularis; VT, 

 ventral thalamus; III, oculomotor nucleus. 



The chimaeras lack pallial formations bridging the two telencephalic hemi- 

 spheres (Figures 9, 10)— a feature that characterizes all living elasmobranchs 

 (Northcutt 19776). Chimaeras may also lack a specialization of the telen- 

 cephalic roof, termed a central nucleus, that is characteristic of all elasmo- 

 branchs. However, it is also possible that they possess a similar but 

 independently derived pallial specialization. The confusion results from the 

 lack of experimental details concerning telencephalic organization in 

 chimaeras and will be discussed in more detail in a subsequent section. 

 Finally, all chimaeras possess a specialized elongated telencephalon medium 

 (tm, Figure 8) whose length may be almost half that of the entire brain. 



The exact intergroup relationships among elasmobranchs are in dispute 

 (Compagno 1977), but four distinct groups can be recognized: squalomorphs 

 (hexanchiform, squaliform, and pristiophoriform sharks), batoids (skates and 

 rays), squatinomorphs (angel sharks), and galeomorphs (some 73% of the 

 living sharks, including lamniforms and carcharhiniforms). 



Sharks exhibit a wide range of brain variation, but this variation generally 

 reveals two major patterns of organization. Hexanchiform, squaliform, 



