474 Bashford Dean Memorial Volume 



toward the meson or middle line, so as to meet the corresponding part of the other side; in 

 the lamprey the cerebral extensions are away from the meson; in the Dipnoi, as shown by the 

 speaker in 1887, they are downward, while in the ordinary and higher air-breathing verte- 

 brates, reptiles, birds and mammals, the cerebral hemispheres expand mostly upward. It 

 is as if nature had experimented in the four directions at right angles with one another from 

 the primitive condition, nearly as in Chlamydoselachus, where the extension is almost uni- 

 formly in all directions from the olfactory axis. ... In this connection the speaker reiterated 

 his previously expressed conviction that in evolution the olfactory portion of the brain had 

 preceded the cerebral; that the ancestral vertebrates needed to smell rather than to think; 

 that the organ of forethought had been, so to speak, an afterthought, and that the cerebral 

 region, so preponderant in man, was rather an offshoot from the olfactory region, and had 

 been interpolated between that and the hinder portions of the brain. 



Hawkes' (1906) figures (my Figures 13 and 14, Plate IV) representing dorsal and 

 ventral views of the brain of Chlamydoselachus are not well adapted for showing the 

 form of the brain, since each figure shows only a lateral half and some parts have been 

 cut away. In general, the brain appears broader and shorter than in the other figures, 

 and the breadth is particularly noticeable in the region of the medulla. Hawkes' descrip- 

 tion (p. 987) of the brain follows : 



The external features of the brain [of Chlamydoselachus] having a typical arrangement, 

 need not be described. . . . Two points only may be noticed: (1) there is a large rhinocoel 

 extending to the end of the olfactory stalk; (2) the dorsal roof of both prosencephalon and 

 rhinocoel is non-nervous. This second point is of considerable interest, as it recalls the 

 condition of Ammocoetes and of the teleosts. The non-nervous roof may be regarded as prim- 

 itive when compared with that of Ammocoetes, but as specialized when compared with 

 that of the Teleosts. That a non-nervous roof should be found among the Elasmobranchs 

 is a point of considerable interest, although its significance is as yet undetermined. 



It is not clear whether Hawkes made a microscopical examination of the roof 

 described as non-nervous; she states merely that this observation was made on an 

 immature specimen. 



AUis's (1923) artistic portrait of the brain of Chlamydoselachus is reproduced as 

 my Figure 7, plate III. This figure gives the impression of being accurately drawn from 

 a well-preserved specimen, and is evidently not in any sense a diagram. It should be 

 explained that the membranes enclosing the brain had not been removed. AUis states 

 that this dissection had not been completed nor controlled when work was stopped by 

 the death of his assistant, Mr. Nomura. Comparison of this figure with Daniers (1934) 

 figure representing a dorsal view of the brain of Heptanchus (reproduced as my Figure 

 28, plate VII), gives point to Carman's remark that the brain of Chlamydoselachus 

 closely resembles that of a notidanid. In Allis's figure, the optic lobes seem considerably 

 smaller, and the cerebellum larger, than in Heptanchus. The olfactory lobes are longer 

 than those of Heptanchus, though Carman says that they are shorter than those of Hexan- 

 chus. These comparisons are of course based on the proportional size of each part in 

 relation to the total size of the brain. The olfactory tracts diverge more strongly in 

 Chlamydoselachus than they do in Heptanchus. 



