126 ME. W. K. PAHKEE ON THE STEUCTTJEE AND 



acquired an elegant fan-shaped form behind ; the notochord (Plate VIII. fig. 10, n.c.) 

 has its anterior two-thirds enclosed in bone (basioccipital), from the fusion of the two 

 laminae. The two laminse of the basisphenoid (Plate VIII. fig. 10, b.s.) have coalesced, 

 and the clinoid regions are becoming bony ; the alisphenoid (a.s.) is entirely ossified. 

 The posterior part of the anterior vertical canal (Plate VIII. fig. 10, a.s.c.) is enclosed 

 in the superoccipital, and the ampulla is now embodied in the " prootic" (pro.), which is 

 bilobate, and which commenced on the supero-anterior thick edge of the auditory capsule : 

 beginning at a selvedge, it was most probably single from the first. The opisthotic 

 (Plate VIII. figs. 8 & 10, op.) is five or six times the size we saw it in Struthio, B. ; its 

 outer lamina (fig. 8) is an inverted crescent; its inner (fig. 10) is semicircular; they are 

 quite distinct. A very large mass of occipital and auditory cartilage (Plate VIII. 

 figs. 8, 9, 10) is still in a soft state, as is all the rest of the cranio-facial axis, except the 

 ethmoid. The middle or perpendicular ethmoid (Plate VIII. fig. 10,p.e.) has an ossi- 

 fication which is roundish, and is nearly 4 lines in diameter ; its two laminae (right and 

 left) have coalesced by a small internal isthmus (Plate X. fig. l,p.e.); and the edges 

 are creeping in all directions on and into the cartilaginous mass. 



It may be remarked here that this mode of diaphysial ossification, — viz. by two external 

 laminae, where there are two free surfaces to the cartilage, by one where there is only one, 

 by a bilobate ossicle when the bone begins on a selvedge, and by a ring in cylindrical 

 rods — is never departed from (as far as I have seen) in the oviparous vertebrata. 



In by far the greater number of instances the process is the same amongst the Mam- 

 malia ; but in the Fish (Teleostei) the retention of a pith of unchanged and constantly 

 proliferating cartilage is the rule. This is not a gathering together into definite bone- 

 plots of the tiny ossicles which are scattered broadcast over the cartilage in the Plagio- 

 stomes, for their bone-grains are intercellular. The thin bony sheath (but for sectional 

 views) might easily be mistaken either in the adult fish, or in the embryos of the higher 

 group, for a splint bone, especially as it often appears in cylindrical bones on one side 

 at first, that side being the freest and most exposed. It must also be held in mind that 

 there is a stage in which the new bony matter has not as yet touched the cartilage-ceDs, 

 and that this is a persistent condition in certain delicate minute fish, e. g. Gohius minutus. 

 Returning to the ethmoid of Struthio, C, we find a condition which is apparently unique ; 

 for the ethnoidal alae which turn over to form the feeble cribriform plate are not ossified 

 separately as in most of the typical birds, the middle plate in them being entirely com- 

 pleted by the vertical piece, but a bone begins at the top of the broad surface, oval in 

 form, and having only this upper lamina (Plate VIII. figs. 3 & 10, eth.). The upper 

 part of the prefrontals, at least, are thus connate; but the "pars plana" generally has 

 its own centre, or centres, even in the Struthionidae. 



The cartilage between these two ethmoidal bones in Struthio, " C." is only a line deep, 

 and coalescence soon takes place. No other bone appears in the cranio-facial axis ; for 

 these two azygous bones, after they anchylose, spread forwards through the whole of 

 the septum nasi, which in this abnormal group is not difierentiated from the middle 



