1889.] NATURAL SCIENCES OF PHILADELPHIA. 25 



otherwise in nascent cartilage, though quite dense and on the verge 

 of ossifying. This internal view of the cranial casket, reveals the 

 fact that at this stage at least the periotic mass is but slightly eleva- 

 ted above the general level of the floor of the brain-case, and in no 

 way as prominent as the external auditory bulla on either side. It 

 still, however is more or less rounded, and it is only later in life 

 apparently that this projection becomes somewhat angulated as we 

 see it in the adult skull. In this region in the cranium of the em- 

 bryo Neotoma the usual vascular and nerve foramina can be easily 

 made out. 



Already in another paragraph, I have alluded to the state in which 

 we find the basi- and presphenoid (Plate II, Fig. 6, b. s. and p. s.) 

 and it will be seen that the anterior part of the cranial floor is fairly 

 well-paved by two other ossifications of mammalian skull. I refer 

 to the rather large and squarish alisphenoids (al. s.), one on either 

 side of the first-named element, and the more oval orbitosj)henoids, 

 one on either side of the presphenoidal cartilage. These bones are 

 already well-ossified and are pierced by the usual nerve foramina ; 

 the latter by the optic (II), and the former by the third branch of 

 the fifth (V 3 ) through the foramen ovale. A slight pituitary depres- 

 sion is to be observed at its most usual site upon the presphenoid. 

 The arrangement of these parts then, with its true stirrup-shaped 

 stapes, indicates that Neotoma holds quite an exalted position, struct- 

 urally, among the eutherian mammalia. 



Beyond the sphenoidal region the endocranium gradually but 

 rapidly narrows, and just posterior to the nasal structures and cham- 

 bers we meet with a sub-vertical cribriform plate (cr.p.) showing mi- 

 nute perforations for the passage of the nerves. Mesially, a posterior- 

 ly rounded crista galli (Plate II, Fig. 5, cr. g.) is to be seen. These 

 parts are as yet all performed in cartilage. 



By carefully removing the investing bones from the fore part of 

 the skull we at once bring into view the chondrified elements of 

 the nasal organs. We have already alluded to the vomer, and now 

 it can be easily detached from the cartilaginous lamina perpendicu- 

 lnris, with which it is articulated by a longitudinal median groove 

 traversing its entire superior margin. 



The base, for the vomer at this stage is entirely in bone, is some 

 3 cms. long, and sharp, or more or less sharp, along its inferior edge. 

 As in most rodents the lamina perpendiculars is produced forwards 



