24 PROCEEDINGS OF THE ACADEMY OF [1889. 



noicl. This latter (bs) is at present but a small, squarish piece of 

 bone occurring in the basic cartilage, just posterior to the pterygoid, 

 which in the skull of an old iV. cinerea, is wedge-shaped in outline 

 for its exposed surface, and stands between the pterygoids, the nar- 

 row end to the front, separated by a distinct suture from the pre- 

 sphenoid, and the broad end behind, separated by a similar suture 

 from the basi-occipital ; the three bones, thus continuous, making a 

 characteristic area having the form of an acute isosceles triangle. 

 Nearly every trace of the notochord has disappeared in our embryo 

 Neotoma at this stage, its former presence being but faintly indicated 

 by a whitish line traversing the basi-occipital plate in a medio- 

 longitudinal direction, and entirely disappearing near its middle. 

 Beyond, the presphenoid shows commencing osseous deposit in a 

 narrow line down its length, but is still chiefly performed in cartil- 

 age, the former being barely perceptible. Referring again to skulls 

 of the N. cinerea, adult specimens, it becomes worthy of remark, that 

 the supraoccipital region and the foramen magnum are both in near- 

 ly the same plane, it being quite vertical, and almost at right 

 angles with the horizontal plane in which the interparietal and 

 parietals lie. This part of the cranium in our embryo, as already 

 stated above, is more or less rounded as shown in the figures. To 

 the outer side of either exoccipital is seen a distinct and spine-like 

 paroccipital, which feature I fail to find in so early a stage as the 

 embryo before us represents. 



Professor Parker in his famous work upon the ' Morphology of 

 the Skull ' in alluding to the development of the pig at its ' fourth 

 stage,' contends that there the notochord is not yet quite obliterated 

 in the basioccipital, though it is rapidly becoming so. He also 

 points out that a separate ossific centre, in that animal, is to be 

 found in either massive condyle, but they soon coalesce with the ex- 

 occipitals on either side. Agreeing with Neotoma fuscipes, the supra- 

 occipital in the pig at this stage is in two pieces, or " patches" as 

 Professor Parker expresses it, and they " run into one another in a 

 day or two " (p. 288). 



Of the three bones that unite to form the periotic ossification, I 

 find but one that as yet appears in any Avay advanced beyond a car- 

 tilaginous condition ; and this is the opisthotic (Plate II, Fig. 5, o.p.) 

 After the membrane bones which form the vault of the skull have 

 been duly removed, this auditory osseous element may be detected 

 posterior to, and to the other side of the periotic capsule, which is 



