16 PROCEEDINGS OF THE ACADEMY OF [1889. 



In our embryo the nasals have advanced to no inconsiderable 

 degree of calcification after the manner of the membrane bones 

 among the higher mammalia generally. By chafing their superior 

 surfaces with the edge of the scalpel the grit of the ossificatory state 

 can at once be detected, though they are yet thin and easily punct- 

 ured. Each one is in contact, laterally, for its entire length with 

 the premaxillary of its own side. In front they overlap the cartila- 

 ginous snout, while posteriorly they are separated from the frontals 

 by a membranous suture, the coronal suture of anthropotomy. They 

 curve downwards mesially, and are in contact with each other for 

 their entire lengths. 



A frontal, as another investing membrane bone, seems to be rather 

 further advanced towards ossification than we found a nasal to be, 

 and this pair of bones are, as before remarked, the chief roofing ele- 

 ments at this stage of the middle area' of the skull. Behind and 

 towards the middle line each one presents a rounded angle to the 

 " fontanelle " (fig. 2, fo.), and is separated from the parietal of its 

 own side by a membranous interval. Within either orbit a frontal 

 is in contact with the nascent maxillary margin in front and below, 

 while mesoposteriorly, and at the side, the bone is juxtaposed to the 

 growing margins of the ali- and orbitosphenoids and the squamosal. 

 (Plate I, Fig. 1). 



Mesially, the parietals are well separated from each other by the 

 intervention of the " fontanelle " (Plate I, Fig. 2, fo.). Here, they 

 are at this stage antero-posteriorly rather narrow, but each one be- 

 comes gradually broader as it proceeds outwards and curves down- 

 wards towards the lateral aspect of the cranium, where the bone is 

 separated by a considerable membranous sutural interval from the 

 squamosal for its anterior marginal moiety ; its posterior border 

 jutting freely into the unossified tract above and behind the auditory 

 bulla of the same side. Either parietal shows even better evidence 

 of ossification than the frontal in front of it, being best marked at the 

 middle of the bone, while the periphery is yet largely membranous 

 which fact no doubt allows the bones to assume the position they 

 eventually attain to in the skull of the adult Neotoma. 



There is a large and distinct interparietal, which the writer is 

 inclined to think ossifies from two centres, one on either side of the 

 middle line, though at the present stage it is one piece, and more 

 thoroughly ossified than any of the elements described in the last 

 few paragraphs (Plate II, Figs. 1, 2 and 4, i. p.). It has the 



