ANATOMY OF A CYCLOSTOME BRAIN 645 



Sulcus hypothalamicus. Johnston ('12, p, 349) describes our sul- 

 cus medius as part of a crescentic sulcus hypothalamicus extend- 

 ing from the interventricular foramen into the infundibulum. 

 From the limited data now available it is by no means certain 

 that the sulcus hypothalamicus which Johnston describes in cy- 

 clostomes is strictly comparable with his amphibian sulcus of 

 the same name (sulcus diencephalicus ventralis of Herrick). In 

 our model of Ichthyomyzon Johnston's sulcus hyothalamicus is 

 completely interrupted between its horizontal and vertical limbs 

 by the lobus ventralis thalami, and on the right side the vertical 

 limb is represented by two distinct sulci, of which the more pos- 

 terior corresponds with part of Johnston's sulcus hypothalamicus. 

 We have designated these vertical grooves as sulcus hypothala- 

 micus 1 and 2 respectively. On the left side of our model the 

 relations are more like Johnston's model, the sulcus hypothala- 

 micus 1 being fused below with sulcus hypothalamicus 2. How 

 significant these individual variations may be it is impossible to 

 determine fron the limited material at hand. 



Sulcus ventralis. This is a short longitudinal sulcus extending 

 between the tuberculum posterius and the chiasma ridge, which 

 was mentioned by Herrick ('10, p. 470). Johnston ('12) failed 

 to distinguish it and considered it a part of his sulcus hypothala- 

 micus; but our model shows it clearly on both sides as a well de- 

 fined groove which crosses the hypothalamic sulci at approxi- 

 mately a right angle. 



The optocoele is a deep lateral depression of the ventricular 

 surface formed by the lateral evagination of the entire wall of 

 the tectum mesencephali, its middle part reaching ventralward 

 as far as the sulcus limitans.. Its posterior wall is formed b.y 

 two eminences, above by a thick tectal swelling which contains 

 the posterior tectal commissure and below by a posterior teg- 

 mental swelling which crosses the site of the sulcus limitans and 

 in this region obliterates this sulcus on the ventricular surface. 

 The optocoele is bounded in front by the massive posterior 

 commissure and a postcommissural ridge which is visible on both 

 the ventricular and the lateral surfaces of the brain. Beneath 

 the anterior part of this eminence and the recessus metathalam- 



