PAPERS OX BIOLOGY AND AGRICULTURE 



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erh.) backward, and the two together mark the ventral 

 border of pyriform cortex. 



The median surface (Fig. 4), known only in recon- 

 structions (since the brain was cut entire), strikingly 

 conforms to the simplicity of the exposed surfaces. It is 

 typically marsupial, but the dorsal or hippocampal com- 

 missure (com. d.)» which assumes a more or less pro- 

 nounced bilaminar form in marsupials, is here triangular, 

 with only a hint of bilaminarity. In this respect it ap- 

 proaches nearer to the solid rounded monotreme form in 

 Coenolestes than it does in the other marsupials. 



T7a 4 



fs. hip 



Fig. 4. Median section of the cerebral hemisphere of Coenolestes obscurus, 

 as reconstructed from serial sections. The thalamus has been 

 cut away. The gyrus dentatus appears on the exposed median 

 surface between the hippocampal and flmbrio-dentate fissures 

 (the latter of which is posteriorly really an alveo-dentate fissure 

 in this case, since the extra-ventricular alveus covering the hippi- 

 campus inversus or upturned median flap of the amnion's horn, 

 intervenes between this part of the fissure and the fimbria). 

 Reference letters: alv., extraventricular alveus; amg., amygdaloid com- 

 plex; b. oh, olfactory bulb; eh. op., optic chiasma ; com. d., dorsal com- 

 missure; com. v.. ventral commissure; fim., fimbria; fs. amg., amygda- 

 loid fissure; fs. fim-.-d... flmbrio-dentate fissure (alveo-dentate fissure 

 posteriorly); fs. rh., rhinal fissure; g. dent., gyrus dentatus; inf., in- 

 fundibulum ; lob. p., pyriform lobe; h t., lamina terminalis ; tub. oh, 

 olfactory tubercle. 



Two great arched fissures mark the position of the hip- 

 pocampus (hip.), which occupies a great part of the 

 median wall of the hemisphere. Anteriorly the upper or 

 hippocampal fissure (fs. hip.) extends very far forward, 

 in the manner characteristic of this type of brain. Pos- 

 teriorly it is considerably less developed than in the 

 opossum, and apparently even less than in Xotoryctes. 

 No suitable figure of Perameles was available for com- 

 parison on this point. Internally this fissural situation 

 is correlated with an extremely simple condition of the 



