368 DB. G. ELLIOT SMITH ON 



resemblance to the combined lohulus centralis and tlie ciilmen monticnM of the human 

 brain, and is quite as com]ilex and built ujion the same morj)holo;5ical plan in Orycte- 

 rojjus as it is in Man. We find that the area which lies behind the fissiira prima 

 {prceclivctUs of Man) is strikingly similar in the two forms. This large area is divided 

 into four primary divisions in both cerebella. These fissures are respectively (adopting 

 Schafer's nomenclature in Quain's 'Anatomy ') the sulcus 2n'<xgracilis, corresponding to our 

 fissure c; the sulcus prcepyramidalis, corresponding to owr Jtssirra secimda; and the sulcus 

 postnochilaris, corresponding to our fissure d. The resemblance between the several areas 

 into which these fissures subdivide the region behind the Jissiirn p)rinia is extremely 

 close, and is carried even further than the general shape of the main divisions. The 

 large triangular mass between the Jissura prima and the fissiire c is subdivided into three 

 parts by the relatively shallow fissures a and b, just as the analogous region in the human 

 brain is split up into the cUmis, the folium cacuminis, and the tuber valvulce by the sulcus 

 postcUvalis and the sulcus Iwriznntalis magnus. 



The area included between the fissures c and d in the brain of Ortjcteropus presents 

 exactly the same shape which has earned the name j:)^'*'''"'^*^^ for the apparently analogous 

 region of the brain of Man. 



It is unnecessary to do more than refer to the uvula nxi(inodMlui<, for they are obviously 

 homologous in the two cerebella. 



In the whole section, then, we find a most striking and surpiisingly close resemblance 

 between the two cerebellar types. And, even more surprising still, the degree of com- 

 plexity of the mesial section in Orycteropus is certainly no less rich than it is in Man. 

 It is uot surprising, when we find this parallelism between two such distantly-related 

 foi'ms as Orycteropus and Homo, to find that the same pattern is widely prevalent 

 among the lai'ger mammals, being found equally in the Carniwra, TJngidata, and Myrme- 

 cophogldce. 



A mere resemblance between two sagittal sections, even when it is so close and so 

 apparently beyond question as are those of Orycteropms and Jlomo, is not, however, a 

 sufficient guarantee of the morphological identity of the similar parts. We need some 

 further corroborative evidence before we can admit their homoplasy. This evidence is 

 to be sought in the behaviour of the lateral extensions of these central parts in a series 

 of cerebella, and also in the develojmiental history of the parts in the bi-ains under 

 comparison. 



The behaviour of the lateral extensions of the lobas auticus and the lobus posticiis 

 arapljf confirms their suggested homologies in the two forms, and although we cannot 

 siTpply the embryological evidence to sujiport this view, we can have little hesitation in 

 adopting it as accurate. 



While there can be little doubt as to the homologies of most of the segments which 

 present an analogous arrangement in the two sagittal sections, we must confess to grave 

 doubts as to the homology of the j^yramid of Man with the part we have tentatively called 

 by this name in Orycte7'opus. If w^e admit that the appearance of a sagittal section is 

 valid and sutficient evidence, Ave would not hesitate to state the homology at a glance, 

 for the disposition, the relationships, and even the shape of the two parts are exactly 



