NO. 4 AN ENDOCRANIAL CAST — GAZIN 5 



approach the alignment seen in modern lemurs. Perhaps a more 

 plausible suggestion would be that the anterolateral depression 

 paralleling the outer margin represents the suprasylvian sulcus, which 

 in the absence of a sylvian or pseudosylvian fissure might be relatively 

 undeflected. In a possibly comparable situation, but in a more complex 

 setting Tilney (1931, fig. 32) has interpreted a pair of forward con- 

 verging fissures on a oreodon endocast as the lateral and ectosylvian. 

 Nevertheless, reviewing Edinger's (1948) presentation of the endo- 

 cranial casts of early horses, something of a parallel is exhibited in 

 Oligocene and somewhat later forms with relatively complex patterns 

 wherein the sulcus cruciatus extended anteromedially from the median 

 part of the suprasylvian, but generally, as in Mesohippus, the lateralis 

 falls short of a union with it. Early horses, however, reveal such 

 differently proportioned endocasts, indicating relatively elongate fore- 

 brains, that comparison here with a much shortened endocast and in 

 an entirely different mammalian line leaves much to be desired. 



Implications as to development or progress in the Smilodecles 

 brain that are possibly evident from the cast representation of the 

 neopallium seem in part correlated with osteological structures. For 

 example, the strikingly expanded temporal lobes would among other 

 things suggest significant neocortical location of the acoustic centers. 

 The importance of hearing is, of course, clearly indicated in the 

 tympanic portion of the skull. Moreover, early expansion, for middle 

 Eocene time, of the occipital lobes of the mantle well over the mid- 

 brain would seemingly point to increasing relative importance for the 

 cortical area related to vision. The large forward facing orbits, an 

 adaptation to an arboreal habit, testify to the importance of this 

 sense. Other implications of neocortical expansion are, of course, not 

 evident in bone structure, but no doubt dexterity in an arboreal 

 habit is critically involved, although it is understood that correlation 

 is largely a function of the cerebellum. 



Cerebellar portion. — The cerebellar portion of the Smilodecles 

 endocranial cast, as already noted, is anteroposteriorly short and 

 noticeably narrower than the cerebral portion. The vermis cerebelli 

 extended prominently above the general level or surface of the pos- 

 terior lobes of the cerebrum, somewhat as in Lemur but with rela- 

 tively a little greater development. No fissura prima is evident al- 

 though such a structure might be suggested by a slight imperfection 

 across the vermis. This is a trace of the parieto-occipital suture which 

 extends laterally on the cast just anterior to the lateral lobes of the 

 cerebellum. There appears to be a slight offset or fracture of the 

 above along this sutural surface. On a second specimen (U.S.N.M. 



