494 SCHULTE, SET WHALE. 



interposed. The line of union is interrupted by the opening of the jugular foramen. Mesally, 

 the capsule extends under the basi-occipital and basi-sphenoid, in such a way as greatly to nar- 

 row the ventral surface of these structures as compared to the dorsal. Although the contact 

 is intimate there is no real union of substance, except at the most rostral part of the line, where 

 there is found the broad basi-capsular commissure already mentioned in the description of the 

 cranial cavity as a whole. It should be noted that this commissure does not affect the whole 

 thickness of the apposed structures, but is merely a thin shell of cartilage joining their ental 

 edges. So that here also the basi-capsular fissure is all but complete. 



DeBurlet in a 104 mm. embryo of B. rostrata ( = acuto-rostrata) , found five commissures 

 present in the course of the basi-capsular fissure. In each case the union of substance involved 

 only the ental surface, the fissure being deep and continuous in ectal view. On the basis of a 

 single embryo deBurlet found himself unable to decide whether the union between basal plate 

 and otic capsule was in process of formation or of resolution, inclining to the latter view, because 

 of the freedom of the periotic in the adult, an opinion which receives support from the further 

 reduction of the commissures in this older foetus. 



Rostrad the otic capsule enters into the border of the fenestra sphenoparietalis. Here its 

 pole is received into a concavity of the external pterygoid, 1 which on its lateral aspect is drawn 

 out into a stout conical process under cover of a process of the squamosal, which forms the mesal 

 boundary of the foramen ovale and in its position and relations evidently corresponds to the 

 processus falciformis of Eschricht 2 and Beauregard. The process of the pterygoid is of late 

 development, for it is not represented in deBurlet's model. 



In the lateral wall of the cranium, the capsule is largely under cover of the squamosal, a 

 small oval area alone appearing in the interval between this bone and the exoccipital. In the 

 natural condition of the parts, this surface is covered by a thick connective tissue, which further 

 closes the gap above the capsule between the exoccipital, the squamosal and the parietal. 

 While this area corresponds in a general way to the elongated mastoid of the adult, it is to be 

 noted, that in this foetus, the relief of the posterior semicircular canal is visible on the surface 

 and the definitive mastoid is as yet barely indicated. 



Under cover of the squamosal, the otic capsule still retains its continuity with the primitive 

 cartilaginous lateral wall, a well marked commissura orbitoparietalis, extending as a horizontal 

 strip from the ala orbitalis to the capsule, which it joins at the origin of the commissura prae- 

 facialis. 



The ventral surface of the capsula otica is concealed by the fibrous auditory bulla, in the 

 substance of which the tympanic is embedded. As in other mammalian chondrocrania, when, 

 as yet in this region, ossification has not begun, two portions of the otic capsule are dis- 

 tinguishable, the pars canalicularis and pars cochlearis, of which the latter is remarkable for its 

 disproportionately large size. 



Pars cochlearis: - - The pars cochlearis has the form of a circular disc with a slightly concave 

 ental surface, and convex ectal surface. The axis about which the coils of the cochlea turn is 

 almost vertical, pointing from above downward and slightly outward. This causes the surfaces 

 to face dorsad and ventrad. The vertical direction of the cochlear axis is an indication of the 



1 Beauregard. Journal de 1'Anatomie et de la Physiologic, V. 29, 1893. 



* Eschricht, D. F., og Reinhardt, J. Om. Nordnvalen (Balaena mysticetus) Klg. Danskevidensk. Selek. Skriftet (5), nat. og math. 

 Afd., V. 



