NO. 4 AN ENDOCRANIAL CAST — GAZIN 3 



DESCRIPTION OF THE ENDOCRANIAL CAST 



The Smilodectes endocranial cast is short and relatively very broad, 

 but no doubt the most striking feature is the remarkable development 

 indicated for the cerebral cortex or neopallium, relatively greater than 

 in any other middle Eocene mammal for which such information is 

 available. Its surface extent, allowing for marked differences in 

 proportions, is nearly comparable to that in the modern lemurs and the 

 lateral extent is entirely comparable, exclusive of folding in such 

 forms as Lemur. Except possibly for a very narrow band adjacent 

 to the anterior poles of the mantle, the rhinencephalon would not be 

 visible dorsally. The temporal lobe of the neopallium extended so 

 far ventrally that in lateral view only the tip of the pyriform lobe 

 would be visible. The posterior poles extended well over the mesen- 

 cephalon, nearly or quite to the cerebellar portion, but as in Lemur the 

 vermis and lateral lobes were not overlapped by the cerebrum. The 

 vermis and lateral lobes of the cerebellar portion of the brain were 

 prominently developed, but the cerebellum in general was short and 

 relatively narrow in comparison with the cerebral width. 



Cerebral portion. — Concerning ourselves for the most part with 

 such details of the neocortical surface that can be inferred from the 

 cast, it is observed that the position indicated for the rhinal fissure 

 is very low on the side of the specimen. Posteriorly it would apparently 

 correspond with the depression occupied by representation of a vas- 

 cular sinus that extended forward through the cranio-orbital foramen 

 into the orbital fossa (Gazin, 1958, p. 37), much as seen in endo- 

 casts of modern Lemur. Somewhat forward the fissura rhinalis would 

 apparently rise above the level of the vascular sinus, extending anter- 

 omedially, then outward and dorsally as well as forward. At this 

 point it should be noted that there is little or no evidence of a sylvian 

 or pseudosylvian fissure, so that about even with the vallecula sylvii 

 the rhinal fissure, lacking this flexure, turned decidedly outward and 

 upward toward the dorsal surface, rather than more forward as in 

 modern forms. This abrupt rise is, of course, correlated with the 

 shortness of the forebrain. The anterior margin of the neopallium on 

 the dorsal surface is indistinctly shown on the cast but evidently the 

 frontal lobes extended forward nearly to the narrowly constricted 

 portion of the olfactory peduncles. Posteriorly there is no evidence 

 of a floccular notch in the neopallium and the posterior or occipital 

 lobes extended essentially or nearly to the lateral lobes of the cere- 

 bellum. Between the posterior poles on the cast a "V"-shaped inter- 

 val anterior to the vermis exposes representation of the longitudinal 



