BRAIN OF FUR SEAL. 23 



terms, "anterior," "posterior," "superior," and "inferior," terms of more universal 

 applicability— "cephalic," "caudal," "dorsal," and "ventral." For "cephalic" and 

 "caudal" Professor Wilder has recently suggested "praeal" and "postal" as equiva- 

 lents; and for "cephalad" and "caudad," "praead," and "pbstad." 



Where certain of the fissures or gyres have beea submerged for a portion or the 

 whole of their course, they have been designated as such, or the equivalent terms, 

 "subflssure" or "subgyre," proposed by Wilder, have been used. 



In the study of the fissures mere surface appearances are not accepted as final. 

 A fissural entity is not always easy to define. The best apparent guide is the relative 

 depth throughout the course of the fissure. We may commonly assume that the 

 greatest depth is at about the middle of its length, and that it becomes gradually 

 shallow toward each end until it reaches the surface, Such a simple condition, 

 however, does not usually exist. One fissure may join the end of another, giving 

 the appearance at the surface of a long, continuous fissure. By separating its walls 

 or "sounding" its depth the true state of afiairs is easily perceived. The presence 

 of a shallow, whether it be near or at a distance from the end of a fissure, would 

 seem-to indicate that at some time during development this shallow has been or will 

 be represented at the surface and serve to separate two independent fissures. 



CALLORHINUS URSINUS. 



Cranial nerves. — The cranial nerve roots of Callorhinus are well developed and need 

 no special comment. In the case of the optic nerves we do not find the X-shaped 

 chiasma, as in Phoca, but the nerves run parallel to each other for a short distance 

 from the chiasma before diverging toward the eyes. 



The third pair or oculomotor nerves have a straight lateral direction from their 

 apparent origins, but at the lateral border of the hypophysis they bend abruptly upon 

 themselves and proceed cephalad, forming a very distinct right angle. 



The olfactory lobes are fairly weH developed. 



Fissures. — K"o special mention will be made of the gyres (convolutions). These 

 are naturally formed from the involutions of the fissures, and it is believed that a 

 careful description of these furrows will by implication include that of the gyres 

 sufflciently for our present purpose. 



The olfactory fissure is completely hidden by the olfactory crus and bulb; when 

 these are removed a shallow fissure is apparent, which becomes deeper toward the 

 base of the lobe. 



Forming the lateral boundary of the olfactory lobe is the rhinal fissure, which 

 passes in a caudo-lateral direction to the Sylvian. An apparent continuation of the 

 rhinal from the Sylvian is known as the postrhinal fissure. It extends in a meso- 

 caudal direction for a centimeter and a half, stopping just short of the cleft between 

 the cerebrum and cerebellum. A careful examination of the postrhinal shows that it 

 has no connection whatever with the rhinal, but is continuous, superficially at least, 

 with the subfissure (postica) lying in the caudal wall of the Sylvian. This condition 

 also obtains in the adult specimen examined. 



LATERAL ASPECT. 



The Sylvian is a convenient fissure to begin with. Tliere is usually some evidence 

 of it if the brain is at all fissured, and, in the lower animals at least, it forms a center 

 around which the other fissures are more or less regularly arranged. In Callorhinus 



