JIORPirOLOfiY OF THE RRAIX IN THE MAMMALIA. 399 



call "suprasylvia anterior"). Its caudal extremity may or may not be joined to 

 the relatively unimportant postsylvicxu sulcus (/. c, the "suprasylvia posterior" auct.) ; 

 the latter is not only much more variable, hut develops much later than tlie true 

 suprasylvian sulcus. In most Carnivores tlie two sulci are united to form a regular a.rc 

 around the pseudosylvian sulcus. But it often happens in almost every genus, and 

 constantly in Uerpestes and its allies, as well as in many Seals, that the sliprasylvian is 

 )iot joined to the postsylvian sulcus. The morphological value and importance of tin; 

 two furrows is so vastly dilTereut, that 1 have thought it advisable to wliolly discard the 

 common and misleading title " suprasylvia posterior," and return to tlic original nam.; 

 " postsylvia " (Owen), which is both simpler and more accurate, as the further discussion 

 Avill show. 



The ectosylvian sulci are features of little morphological importance, and are to b^ 

 regarded as furrows compensatory to the pseudosylvian sulcus. In many gener.i botli 

 ectosylvian sulci are absent : in flie Ilya^nidfc the anterior ectosylvian sulcus is absent; 

 and in the Bears the two sulci are sul)merged. Even when present the sulci are 

 exceedingly variable. 



Next in importance to the suprasylvian sulcus come tlie orbital (presylvian), coronal, 

 and lateral, and, after a wide interval, the crucial, postlateral, arcuate, and prorean sulci 

 in that order. 



The presylvian (or, as I prefer to call it, "oi-bital ") sulcus makes its appearance at a 

 very early period, and is almost as constant as tlie suprasylvian. It usually becomes 

 considerably prolonged so as to join the rliiual fissure below and so as to extend near 

 to the dorso-mesial edge of the hemisphere above. But both these phenomena must 

 be regarded as secondary modifications, Avliich take ]Aiuie under the influence of purelv 

 mechanical factors, and are therefore devoid of great morphological imnortance. The 

 true fundamental orbital element is a small furrow jjlaced in the neopallium alono-sid** 

 the anterior end of the rhinal fissure : as such it is perhaps the most constant and one of 

 the most stable of all the neopallial sulci in the Mammalia. The coronal and lateral sulci 

 are second in importance only to the suprasylvian and orbital. The former is perliaps 

 veiy slightly the more precocious, but occasionally this sulcus is absent when the lateral 

 sulcus is present, as sometimes happens in Uerpeslcs. [This is also especially the ctise in 

 Rodents and in many Marsupials. On the other hand, the coronal sulcus is often 

 present when the lateral sulcus is absent in some Ungulates, and esjjeeially in FrocavuiA 

 In the Carnivora as a whole these two sulci are very constant, and in most cases they 

 are united to form one furrow. 



The direction of the coronal sulcus is subject to considerable variation in the different 

 families. In tlie larger members, and especially the Ursida^, it becomes almost transverse ; 

 whereas in the simple, generalized Viverridse its direction is almost sagittal. 



A postlateral sidcus is often united to the caudal end of the lateral, but it is exceedingly 

 variable and may be entirely absent. In its simplest form it often consists of a small 

 transverse furrow behind (and not joined to) the lateral sulcus. 



The crucial sulcus, which is often regarded as a distinctively Carnivore feature, is not 

 constant. Its size is exceedingly variable. So large doi's it become in the Bear thai 



SECOND SERIES. — ZOOLOGY, VOL. Vlil. 68 



