ELASMOBRANCH BRAIN ORGANIZATION 145 



the group. Existing data (Bauchot et al., 1976) on the angel shark Squatina 

 would change the lower boundary considerably if included. 



Data on the advanced batoids (myliobatids, rhinopterids, and mobulids) 

 are particularly needed. Several of the myliobatiforms reach body weights of 

 50 to 1000 kg, and thus the present polygon is greatly abbreviated. Future 

 considerations should exclude the data of Crile and Quiring (1940) on 

 Dasyatis sabina. Two different brain weights have been reported for the same 

 specimen (Crile and Quiring 1940, Quiring 1950) and these values are further 

 complicated by possible misidentification of the specimen. Bigelow and 

 Schroeder (1953) claim that D. sabina is the smallest of the North Atlantic 

 stingrays. The largest specimen reported by Bigelow and Schroeder was 39 

 cm wide thus a weight of 18 kg, as reported by Crile and Quiring, is 

 impossible. 



Similarly, the absence of sharks in the upper half of the polygon in Figure 

 19 may be an error due to small sample size. Data on smaller carcharhinid 

 and sphyrnid species might extend the present distribution; such data are 

 needed before it is concluded that the advanced batoids possess higher 

 brain: body ratios than the sharks. 



The common conception that chondrichthians are small-brained creatures 

 is clearly false, but how are the brains of chondrichthians organized? Do 

 these animals possess massive lower brain centers, or do they possess well- 

 developed forebrains like those of birds and mammals? An analysis of the 

 data presented in Figure 1, on the relative development of the major 

 divisions of the brain in a number of chondrichthians, reveals a wide range of 

 variation. 



Species such as Hydrolagus, Squalus, and Platyrhinoidis possess relative 

 forebrain development comparable to that of teleosts and amphibians, while 

 the advanced galeomorph sharks possess relative forebrain development com- 

 parable to that of endothermic vertebrates (Ebbesson and Northcutt 1976). 

 The distribution of high telencephalic percentages in both sharks and batoids 

 suggests that these levels of neural development have occurred independently 

 and that several levels of development exist within both radiations. 



Analysis of brain divisons as percentages of total brain weight or volume 

 indicates which divisions of the brain are highly developed. However, it fails 

 to account for the possibility of independent atrophy or hypertrophy of 

 other brain divisions. Thus, this is not the most accurate assessment of the 

 relative development of brain divisions among various species. A more mean- 

 ingful numerical analysis is obtained by calculating ratios for brain division 

 weight to body weight. Using a "quick" method (Jerison 1973) to correct 

 for body weight with a coefficient of allometry of 0.76 (a determined from 

 sample presented in Figure 19), I have calculated the relative weights (Table 

 3) of a sample of brain divisions of the species listed in Table 4. The figures 

 obtained have been multiplied by 10 4 to avoid decimals. This method is 

 "quick" in the sense that it is a first approximation. Given a sufficiently 

 large sample, it would be possible to determine a coefficient of allometry 

 and intercept for each brain division in both sharks and skates. Since all 



