170 Charles Searing Mead. 



arches (the visceral skeleton), the different parts of the skull are 

 not separated by sutures, the cartilage composing the neural cranium 

 forming one continuous unit. Kolliker has divided the skull into 

 the posterior part, the pars chordalis, through which the notochord 

 runs, and the par's pi^ecliordalis, situated anterior to this. Each of 

 these divisions Gegenbaur has again divided into two regions, which 

 Gaupp has named, from behind forward, as follows : i^egio occipitalis, 

 regio otica, regio orbitotemporalis and regio ethmoidalis. For de- 

 scriptive purposes it will be convenient to follow these divisions, ex- 

 cept that the basal portions of the two posterior regions will first 

 be considered together, since they form a fairly complete unit, the 

 planum basale (Gaupp). 



Planum Basale. 



The planum basale comprises the basal portions of the two poste- 

 rior regions of the skull. It extends from the foramen magnum 

 forward to the posterior border of the hy^wphysial fossa and is 

 perforated for nearly its entire length by the notochord. Its anterior 

 three-fifths, i. e., the portion belonging to the regio otica, is narrow 

 and rod-like (PL I and Figs. 1, 3, 4 and 5), while its posterior por- 

 tion is spread out into a broad quadrangular plate, the basilar part 

 of the occipital region. 



The anterior portion of the basal plate in Sus differs strikingly 

 from that in other mammals on account of its narrow, rod-like shape 

 and also because of the character of its union with the ear-capsules. 

 Here it is separated from the ear-capsules except for a thin connect- 

 ing lamella of cartilage, while in the other mammals that I have 

 examined it is firmly united with the capsules. In the other mam- 

 mals, also, the planum basale forms a broad plate of cartilage, which 

 passes over without sharp demarcation into the broad basal cartilage 

 of the orbitotemporal region. Tarsius offers an exception to this 

 statement, since in it the portion between the cochlear spheres is 

 even narrower than in Sus. This, however, is due to the extremely 

 large size of the ear-capsules, which approach the middle line and 

 compress the cartilaginous plate which lies between them. 



Back of the auditory buUse the planum basale spreads out into 



