100 ILLINOIS STATE ACADEMY OF SCIENCE 



THE BRAIN OF COENOLESTES OBSCURUS 



Jeannette Brown Obenchain, University of Chicago 



Coenolestes is a small ratlike Americal marsupial 

 about five inches in length from tip of snout to root of 

 the slender tail. It is a native of the high Andean forests 

 and has been known to science since 1860, so far by rare 

 and usually incomplete specimens from Colombia, Ecua- 

 dor and Peru. Dr. Wilfred H. Osgood of the Field Mu- 

 seum recently collected eleven specimens, which he made 

 the basis of an extensive monograph published in 1921. 

 The brain of one specimen was sufficiently preserved for 

 study, and Dr. C. Judson Herrick described and figured 

 its dorsal, lateral and ventral surfaces in an appendix 

 to the monograph. Cut into serial sections and stained 

 by the iron-haematoxylin method to show both cells and 

 fibres, it forms the basis of the present study. 



The brain of this creature is especially interesting for 

 two reasons. First, because it is one of the simplest and 

 most generalized of mammalian brains, since both the 

 monotreme brain's are rather highly specialized. Second, 

 because the classification of Coenolestes in one or the 

 other of the two marsupial suborders, Polyprotodontia 

 and Diprotodontia, has given rise to a long and lively 

 controversy. 



In his account of the external features of this brain 

 Dr. Herrick drew attention to its extreme simplicity, as 

 evidenced both by the enormous development of the visi- 

 ble olfactory regions (olfactory bulbs, olfactory tu- 

 bercles, lateral olfactory cortex) and by its small and 

 smooth cerebral cortex, which he thought to be probably 

 the least extensive, relative to the total weight of the 

 brain, in the whole mammalian series, as so far described. 

 He noted also that in external conformation the brain of 

 Coenolestes resembled much more closely those of two 

 Australian polyprotodont forms, the bandicoot Perame- 

 les and the marsupial mole Notoryctes, than it did that 

 of the American opossum. 



The lateral olfactory cortex, the cortex of the pyrif orm 

 lobe (lob. p.), occupies more than half the lateral sur- 

 face of the brain (Fig. 1). Both anteriorly and posterior- 



