AND DEVELOPMENT OF THE BIRD'S SKULL. 147 



cleft. The "hinder angles of the basitemporal plate are greatly produced, forming horns 

 that underlie the exoccipitals, whilst the posterior concave edge has lying on it the fore 

 end of the basioccipital (bo). 



This latter bone has on each side, between it and the paired exoccipitals, a reentering 

 angle of soft cartilage in front ; but behind the three meet in the very substance of 

 the transversely oval occipital condyle; here, in meeting, they form a suture like a 

 leech-bite. 



For the most part the occipital arch is well ossified (Plate XXVII. figs. 1-4) ; but the 

 whole of the outer edge of this transversely crested structure is still unossified, the car- 

 tilage not merely existing on the tympanic wings of the exoccipitals, but also on the 

 lower half of the superoccipital ridge (teo, so). The original suture between the two 

 superoccipitals is still visible above; but the " lateral occipital fontanelles " are nearly 

 filled in by bone on each side the narrow waist of the median part of the keystone. 



We note here already the partial obliteration of the two very important Pluvialine 

 characters in these Sea-Mew chicks, namely, the basipterygoids and these occipital 

 windows. Here specialization has taken place in two ways : — first, by arrest and absorp- 

 tion; and secondly, by the secondary covering in of a region not filled in by the 

 chondrocranium. 



The next important point to be noticed in these chicks of the third stage is the recol- 

 lection, as it were, of the epiotics left out in the first growth of bony territories ; they 

 are, however, very small, and broken up into two or even three subcentres (Plate XXVII. 

 figs. 2, 5, & 6, ep). 



This is seen many a time in ornithic morphology ; the bones which get the first start 

 in growth are large, whilst those which lose it are small, feeble, and overshadowed. 



The upper view of the skull-base (fig. 2) shows well the round, deep, pituitary cup, 

 pierced below by the internal carotids (ic) ; also the slit below the hinder pituitary 

 wall, which is the old gap caused by the retirement of the notochord. The elegant basi- 

 occipital lozenge somewhat wedges itself into the end of the large, long, compound 

 basisphenoid. 



But perhaps the most notable bones of all are the prootics ; immense are they if com- 

 pared with those of the mammal. They are scooped laterally for the brain, and have 

 a lunate notch in front, which by a similar notch in the alisphenoid (als) becomes the 

 foramen ovale (5). 



The prootics tilt themselves back to such a degree that the great anterior semicircular 

 canal is thrown, at its junction with the posterior canal, into the fore edge of the super- 

 occipital (figs. 4 & 5, asc, so). 



If this figure (2) be compared with the rest, it will be seen how neat is the carpentry 

 by which the great ethmo -trabecular plate rests in the grooved upper surface of the 

 parasphenoidal rostrum (pas). 



There it stands as a fixture ; but this plate, the great meso-ethmoid, is almost sawn 

 through from below in front of the rostrum, and its under beam is the light and loosely 

 braced vomer. 



SECOND SERIES. — ZOOLOGY, VOL. I. X 



