Chap. II. BRAIN IN DIFFERENT FAMILIES. 363 



is compact, like that of a Mammal, and generally very broad ; but the brain-box 

 and the brain arc slender and small, while in all Mammalia and in all Birds, 

 in which latter the skull is often very slender, the brain is broad, short, and 

 high. The large development of the muscles, and especially of the bony frame- 

 work of the head, and not that of the brain, accounts for the broad form of 

 the skull of the Testudinata, the locomotive apparatus of the powerful jaws l)eing 

 chieHy placed on the sides of the skull. As we have already given a brief 

 sketch of the brain of Turtles in general, when treating on their nervous sys- 

 tem,' we have now only to compare the brains of different families with each 

 other. 



In spite of the constancy in the proportions of the brain, in the whole 

 order, some differences may be noticed when comparing singly the parts of the 

 brain of different families with one another. In the first place, it may be 

 remarked, that the two sub-orders described above as Chelonii and Amydas seem 

 as well justified by the peculiarities of their brain as by the other characters 

 they exhibit. In the sub-oi'der of Chelonii proper, the large hemispheres are 

 more cyUndrical, nearly as high as broad, and, without broadening and forming 

 an outgrowing angle behind, they taper into the posterior part of the brain, the 

 corpora quadrigemina ; while, on the contrary, in all the AmydjB, the hemispheres 

 are much more depressed, generally marked with some folds, and always widen 

 backwards, so as to form there an abrupt angle with the rest of the brain. 

 This is particularly the case in Trionychidae, much less so in Chclydroida?, more 

 again in Cinosternoida^, and still more in Emydoidae and in the land Turtles. In 

 this respect the latter, the Testudinina, stand next to the Trionj-chidos, which, as far 

 as this point is concerned, seem to rank first. The large hemispheres are nearly 

 smooth in Trionyx ; in the Emydoids, and still more in Testudo, we see fine folds 

 run along them. The corpora quadrigemiua are largest in proportion to the hemi- 

 spheres, and more longitudinal in Chelonii proper, smaller and more rounded in 

 Amydaj, and often nearly entirely received into the posterior excavation of the hemi- 

 spheres, as in Trionyx. The cerebellum is remarkably high in sea Turtles ; it is flat- 

 ter and thinner, more like a bridge, over the fourth ventricle, in the Amyda). It is 

 remarkably broad in Trionyx and Emys, narrower in Cinosternoida3 and in Che- 

 lydroidae. In sea Turtles, the fourth ventricle is narrow; broader in the Am3-dae, 

 and very wide in land Turtles. In Trionychidse, Chelydroidte, Cinosternoida?, and 

 Emydoida), the whole ventricle has a constant typical shape ; that is to say, it is 

 much more slender when compared with that of the land Turtles, and broader in 

 front; then follows a contraction, when it widens again, and runs out into a long, 



' Comp. Cliap. 1, Sect. 8, p. 274. 



