MORPHOLOGY OP OPISTHOCOMUS CRISTATUS. 51 



showing how high even the lower kinds of Carinatse are, as compared with the cold- 

 blooded Sauropsida ^. 



In these high skulls there is a partial secondary segmentation of the ethmoid from 

 the anterior sphenoid by a pyriform interorbital fenestra (i.o.f.) ; above this the 

 ethmoid ends as a blunt cartilaginous crista galli, under which the olfactory crura (i.) 

 run forwards to the simply infolded upper and middle turbinal regions. Below this 

 groove we see the narrow orbito-sphenoidal alae (o.s.). In all these stages the anterior 

 sphenoid is wholly unossified : in Stage 1 the only part of the posterior sphenoid that 

 has any bony matter is the rim of the pituitary hole ; the trabeculae do not form a 

 floor to this part; and that bony substance is borrowed from the parasphenoid, a 

 mere parostosis. The large leafy postorbital alisphenoids are still thin upgrowths of 

 cartilage (PI. VII., al.s.) ; but in Stage 3 these are largely ossified (PI. VIII. fig. 2, 

 al.s.}, and although turning in behind the eyeballs, do also form the foremost third 

 of the lateral wall of the skull, the rest being made by the superficial low-lying 

 parietals (PI. VIII. fig. 2, ja.). Under these two bones, and wedged in between them 

 and the large occipital arch, we see the large, long auditory capsules, which are so 

 much tilted back as to have their top nearly as low as their base. These cartila- 

 ginous capsules were fused very early with the parachordals, and with their lateral 

 upgrowths, alisphenoidal and exoccipital tracts. 



A considerable osseous centre is already seen in Stage 3 (PL VIII. fig. 2) between 

 the meatus internus (vii., viii.) and the elegant anterior semicircular canal (a.s.c.) ; this 

 is the prootic {pr.o.}. Below, also, over the edge of the exoccipital (e.o.), a lanceolate 

 bone is seen hardening the postero-inferior edge of the capsule ; this is the opisthotic 

 {op.) ; the epiotic is rarely seen in birds, and then only as a small tract of bone. Even 

 in Stage 1 the double supraoccipital (s.o.) has become a single tract of bone ; the 

 exoccipitals are growing well at the sides of the arch, leaving, however, the large 

 tympanic wings (PI. VII., t.eo.) in a soft state ; the basioccipital is still largely hidden 

 in the cartilage (PI. VIII. fig. 1, b.o.) ; it is forming, however, round the sheath of the 

 notochord (nc.) ; the occipital condyle (oc.c.) is slightly bilobate. In Stage 3 (PI. VIII. 

 fig. 2, nc.) the notochord has retreated into the basioccipital region, although at first 

 it reached, at least, the top of the post-pituitary wall. Under that wall, in this advanced 

 stage, much absorption of the basal cartilage has taken place, to form the middle part 

 of the cavities in front of the cavum tympani — the pre-tympanic recesses of the basi- 

 sphenoid. These parts will be described with the parasphenoids ; but before leaving 

 the chondrocranium I must refer to the foramina of the cranial nerves. 



The first nerve (PI. VIII. fig. 2, i.) has already been mentioned ; it is a single crus, and 

 does not need a lamina cribrosa. The optic nerves (ii.) have a large common passage 



' It seems to me that the primary Dipnoan Bird-stock had a face as long as in the Skate ; not short, as in 

 the existing Dipnoi and Amphibia. 



VOL. XIII. — PART II. No. 2. — April, 1891. i 



