ELASMOBRANCH BRAIN ORGANIZATION 169 



The visual system is well developed in Hydrolagus, and the entering optic 

 fibers divide the superficial tectal zone into distinct layers. A thin series of 

 optic fascicles runs horizontally, just beneath the tectal surface; these 

 fascicles are covered by unmyelinated nonretinal fibers, as in elasmobranchs. 

 However, most of the optic fibers enter more deeply. Thus, a distinct neuro- 

 pil of dendritic processes, unmyelinated fibers, and some cells exists between 

 the small superficial optic fiber component and the massive deep optic 

 fibers. This neuropil is probably the major site of optic terminals, but experi- 

 mental studies are needed to confirm this speculation. 



The tectum of Notorynchus is almost identical to that of Squalus (Figures 

 2,3). Both possess deep periventricular laminae (one to two cells thick) and 

 a centrally located cellular plate that is possibly homologous to tectal layers 

 4 and 5 of Mustelus. Clearly most of the tectal neurons are restricted to the 

 central tectal zone, thus differing from Hydrolagus as well as the galeomorph 

 sharks (Figures 4, 5). All galeomorph tecta examined to date, except possibly 

 Scyliorhinus, are similar to Mustelus. The tectum of Scyliorhinus appears 

 intermediate between that of squalomorphs and galeomorphs, but experi- 

 mental study of the retinal projections is needed to confirm this observation. 

 The tecta of Platyrhinoidis and Raja (Figures 20, 27), like those of galeo- 

 morph sharks, have the highest cell concentrations in the superficial zone, 

 and can be distinguished from the tecta of all sharks by a periventricular 

 zone without distinct cellular laminae, and by a more restricted mediolateral 

 axis. However, the same tectal layers can be recognized in batoids as in 

 galeomorph sharks; and the optic fibers predominantly enter the deeper half 

 of the superficial zone and terminate more dorsally (Northcutt and Boord, 

 unpublished observations). 



The range of tectal variation observed in chondrichthians suggests that 

 chimaeras possess the most primitive tectal pattern, and that the tecta of 

 elasmobranchs are characterized by extensive cellular migrations away from 

 a thick periventricular cellular plate. Among elasmobranchs, squalomorphs 

 probably exhibit the most primitive tectal pattern, characterized by a dense 

 cellular plate in or on the border of the central tectal zone. Both galeomorph 

 sharks and batoids possess tecta with hypertrophied superficial tectal zones 

 of high cell density. Additional studies will likely reveal that the advanced 

 tectal condition has evolved independently in galeomorphs and batoids. 

 As the periventricular tectal zone is traced laterally in chimaeras and 

 sharks, the cellular laminae lose their compactness and form a scattered 

 nucleus called the torus semicircularis (Figures 3B, 5). I have been unable to 

 recognize a cytologically distinct torus in batoids, and experimental studies 

 are needed to determine the homologous population. The connections of the 

 torus are unknown in chondrichthians, but in other anamniotes a similarly 

 situated nucleus receives auditory and lateralis afferents (Page 1970, Knudsen 

 1977), and toral efferents have been traced to thalamic and reticular 

 populations. 



The midbrain floor, or tegmentum, passes over from the hindbrain 

 through the isthmus, a transitional region that is sometimes recognized as a 

 separate brain division. The isthmus, or caudal tegmentum (Figure 20 A), is 



