Explanation: of Platks. 
PLATE I. 
Figures 8, 9, and 10 of this plate were drawn by C. L. Herrick. 
Fig. I. Brain of Erethizon dorsatus, ventral view, with the hypophysis, 
without the medulla and cerebellum, twice natural size. 
Fig. 2. Region of the calamus scriptorius of Erethizon dorsatus, cerebeb 
lum removed, twice natural size; r, recessus lateralis; /, pedunculus cerebelli in 
section; t, fasciculus teres; a, eminentia acustica; g wz, genu of the seventh nerve; 
0, obex; cl, clava; c, calimus scriptorius; gr, furniculus gracilis; cu, furniculus cu- 
neatus ; Ic, lateral columns. 
Fig. 3. Brain of the Erethizon dorsatus, hemisected, median view, cerebel- 
lum removed, natural size; c, callosum; f, fornix commissure;/^, praecommissure; 
me, medicomniissure; h, habena; p, pedunculus cerebelli in section; r, recessus 
lateralis. 
Fig. 4. Brain of Erethizon dorsatus, dorsal view, natural size. 
Fig. 5. Brain of Geomys bursarius, ventral view, hypophysis removed, 
twice natural size; the Roman numerals refer to cranial nerve roots. 
Fig. 6 . Brain of Erethizon dorsatus, ventral view, hypophysis removed, 
twice natural size. The exit of the third nerve is abnormally on the lateral as 
pect of the crus in this brain, by reason of the fact that after leaving the brain it 
turns laterad some distance before penetrating the membrane; cf. Fig. i. 
F^g. 7. Brain of Erethizon dorsatus, lateral view, natural size. 
F^g. 8 . Brain of the white rat, dorsal view, natural size. 
F^g. 9. Brain of Geomys bursarius, dorsal view, natural size. 
Fig. 10. Brain of Geomys bursarius, lateral view, natural size. 
PLATE 11 . 
The outlines of ail figures of this plate were drawn with the aid of the cam- 
era lucida. 
Fig. I. Horizontal longitudinal section of the brain of Mus musculus at 
the level of the anterior commissure. 
Fig. 2. Horizontal longitudinal section through the brain of Geomys bur- 
sarius at a level slightly ventrad of the anterior commissure; rxl, radix lateralis, 
or lateral olfactory root; pc, praecommissural tract of the olfactory; op, optic 
tract; f, descending fornix tract entering the mammiliary. 
Fig. 3. Horizontal longitudinal section through the brain of Erethizon dor- 
satus at the level or the anterior commissure, drawn to the same scale as Fig. 13; 
