104 



ANATOMY OF VERTEBRATES. 



degree influences the developement of a supraventricular trans- 

 verse commissure ; the seeming small one exposed at o, fig. 7 1 , 

 is hippocampal or psalterial. This low phase of Mammalian brain- 

 growth is essentially related to the common monotrematous con- 

 ditions of generation. 



The brain bears a small proportion to the body in the Marsupial 

 order; in the Ursine Dasyure, Hg. 72, it is as 1 to 520; in the 

 Wombat, as 1 to 614; in the great Kangaroo, as 1 to 800. In 

 smaller Kangaroos the disproportion is less; thus in the Tree- 

 kangaroo (Dendrolagus inustus) I found it as 1 to 250. The 

 brain is relatively largest in the smaller species of Petaurists and 

 Phalangers. 



The cerebral hemispheres do not extend over the cerebellum 

 in any of the species, and in some, as the Dasyures and Opossums, 

 they leave the optic lobes exposed. In the Phalangers and Petau- 

 rists, the Opossums, Perameles, the insectivorous Phascogales, 

 and the smaller Dasyures, the exposed surface of the cerebral 

 hemispheres is unconvoluted. In the Dasyurus ursinus, fig. 72, 

 b, this surface is broken by a few slight indentations, two of which 

 may indicate the beginnings of the ( medi-lateral ' longitudinal 

 folds. 



In the Wombat an ectorhinal fissure bounds the outer side of 

 the olfactory tract at the base of the 

 brain ; l from the anterior moiety of this 

 fissure three or four smaller ones curve up- 

 ward upon the sides of the hemispheres, 

 one of which answers to the ( fissura Syl- 

 vii,' 2 but is less defined than in the Kan- 

 garoo. On the upper surface a short 

 transverse fissure marks off the outer part 

 of the anterior lobe of the cerebrum, and 

 behind this each hemisphere exhibits a few 

 detached shallow fissures. 



The American Opossums show a range 

 in size from that of a mouse to that of a 

 cat, and the Australian Dasyures rise from 

 the same diminutive extreme (Antechinus 

 pusillus) to the size of the wolf ( Thyla- 

 cinus). But the cerebral hemispheres are as smooth in Didel- 

 phys Virginiana 3 as in D. (Philander, Microdelphys) murina ; 

 and the great Ursine Dasyure, fig. 72, shows but a few short and 

 shallow indentations of the exposed cerebral surface. 4 Thylacinus 



Brain of Dasyurus ursinus. 



UX' pl. V. fig. 8. 



2 lb. fig. 3. 



3 lb. fig. 6. 



* lb. fig. 5. 



