MORPHOLOGY OV THE BHAIX L\ THE MAMMALIA. 403 



depressions representing diagonal*, posterior ectosylvian, and crucial sulci of ilie 

 Carnivore-type. 



The feature of most interest, however, is the ])S('U(losylviau sulcus. Tliis usually 

 consists of a shallow depression above the bend of the riiiual iissure: there is a well- 

 defined posterior lip, but no anterior lip. Eroni the apex of the depression a deep 

 and well-defined sulcus begins and arclics backward close to the upper end of the post- 

 sylvian sulcus. 



The reader will observe that so far no mention has been made of the suprasylviaii 

 sulcus, the most stable sulcus on the outer surface of the hemisphere in the Carnivora 

 and Ungulata. It might, perha])s, be supposed that, as this sulcus is sometimes absent 

 in Tanumdua, it might also be absent, as a rule, in Jlynnecophucja. JUit it is hardly 

 likely that a cerebral hemisphere which is provided with lateral, coronal, orl)ital, post- 

 lateral, postsylvian, pseudosylvian, proreau, and even diagonal, posterior ectosylvian and 

 crucial sulci, should lack just that particular sulcus which is the most constant and stable 

 sulcus in the Mammalia. Such a phenomenon is so highly improbable as to be almost 

 inconceivable. There can be no doubt whatever that the suprasylvian sulcus is present 

 as the deeply-incised, dorsal, arcuate appendage of the pseudosylvian sulcus. 



Eortunately there is an anomalous brain in the Collection of the Royal College of 

 Surgeons [D. 281, Catalogue, p. 22o, fig. 1)5] which proves that this is so. On the left 

 liemisphere of this brain the condition described above is found ; whereas on the right 

 hemisphere the suprasylvian sulcus f is separated from the pseudosylvian sulcus and 

 joined to the jiostsylvian, as in most Carnivores. 



In Orijcteropm the suprasylvian sulcus is only poorly developed, whereas the coronal, 

 lateral, orbital, and ])rorean sulci are exceedingly well-developed. There is no trace of a 

 pseudosylvian sulcus. 



In Manls there are typical suprasylvian, orbital, corono-lateral, and pseudosylvian 

 sulci, as in Brudii])ux. 



In the Armadillos the orbital sulcus is always, and the suprasylvian sometimes present. 



In many of the Rodents there is an extraordinary j)aucity or even absence of sulci. 

 The lateral is perhaps the most constant sulcus, and the orbital and coronal sulci are 

 found in many Rodents. In Hi/drocha'nis aiul Lagodumus there is a su2)rasylviau 

 sulcus ; and in the former numerous other furrows of doubtful homology. The most 

 interesting brain among the Rodeutia is that of Dollcholls. In addition to the typical 

 orbital and sagittally-directed corono-lateral sulcus there is a suj)rasylvian sulcus like 

 that of LagostomH.s, l)ut it is joined to a pseudosylvian sulcus to form a conjoint Sylvian 

 furrow, such as is usually found in the Edentate M'jrmecophaga. These forms are of 

 particular interest in comparison with the condition found in the Lemurs, as will be 

 apparent later ;|:. 



* The diagoual sulcus, both of the liradypodidoc and tlii' MynneropLagidii!, is always dircctod upward and forward 

 as in the Ungulata and Primates, and is not subject to the irregularity and changes iu direction which wc find in tha 

 Carvivora. 



■^ .\ comparison of the two liemispheres shows that the supra.sylvian sulcus is certainly present in both. 



J Sea Catalogue of the Royal College of Surgeons, 2nd Edition, vol. ii. p. 205, fig. 75. 



