116 ME. W. K. PAEKER ON THE STEUCTUEE AND 



been long differentiated from the lateral ethmoid, and the quadrate cartilage from 

 the front, and the stapes from the hinder part of the auditory capsule (Plate VII. fig. 4, 

 pa.-q.). 



The cartilaginous skull of the Ostrich is at this stage perfectly formed ; yet the inter- 

 cellular substance is scanty, and the whole structure is very friable : its shape will 

 undergo but little change. 



Beginning at the basioccipital region, we find a large conical mass of notochord 

 (Plate VII. figs. 1, 2 «& 6, n.c.) reaching only halfway to the deep cavity of the " sella 

 turcica;" this mass is enclosed in the thick investing cartilage, which is deficient at the 

 top and thick below, and has its posterior half enveloped in the large, transversely 

 oval, occipital condyle. In front of this condyle, both above and below, there is a 

 thin layer of bone (Plate VII. figs. 1 & 2, b.o.); the upper layer is encroaching upon the 

 cartilage downwards, and the lower lamina upwards, but there is a thick mass of 

 untouched cells between them. The upper lamina of bone is oval, the lower lozenge- 

 shaped ; the latter reaches in front to a trifoliate mass of cartilage, which has almost 

 filled in the original space between each half of the investing mass, just up to the 

 point where each cranial " trabecula " is given off. There is in front of this secondary 

 growth of cartilage a true pituitary "fontanelle" (Plate VII. fig. 4, ^.^.s.) still remaining; 

 it is oval, and about half a line in fore-and-aft extent. In the cranial floor, however, 

 the cartilage is still more deficient ; and the anterior half of the " investing mass," viz. 

 that which belongs to the posterior sphenoid, is separated by a fissure (Plate VII. 

 fig. 1, h.o.). 



It will be seen in my subsequent papers how all these minute particulars bear upon 

 the structure of the skull in other classes, and how they look back upon the earlier stages 

 of even the ornithic skull. The exoccipital (Plate VII. figs. 1, 2, 4 & 6, e.o.) does not 

 begin at two points, but takes advantage of the cartilaginous selvedge formed by the 

 " foramen magnum ;" it spreads equally inwards and outwards, embracing the thick 

 ascending plate ; the laminae eventually meet each other in the substance of the carti- 

 lage. At this stage the exoccipital is a very small patch, notched in front, where it half 

 encloses the anterior condyloid foramen (Plate VII. figs. 1 & 2, 9.) ; it does not reach 

 the chink for the vagus nerve. The whole of the broad, flat superoccipital cartilage, and 

 the whole of the periotic capsule (Plate VII. figs. 1, 2 & 6, s.o., a.s.c, p.s.c, h.s.c.) is still 

 free from ossific deposit ; the former part is already very thick, but the cartilage covering 

 the large semicircular canals is thin, and reveals their form very perfectly on the wide 

 vertical occipital plane (Plate VII. fig. 6). These are the posterior and horizontal 

 canals, but the largest of the three is the anterior, and this is entirely seen within the 

 cranial cavity (Plate VII. fig. 2); the obliquity of the whole capsule is so great that 

 this canal is tilted backwards into the superoccipital region, and receives, ultimately, 

 much of its bony investment from the superoccipital piece. 



The sinus canal (Plate VII. figs. 2 & 5, s.c), near the junction of the posterior 

 semicircular canal with the upper edge of the superoccipital cartilage, is very wide 



