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Show 1899.] OSTEOLOGY OF THE PYGOPODES. 1021 Ihe Base of the Skull.-The basitemporal plate of the parasphenoid is markedly inflated, and has a bevelled anterior border, the free edge of which, in the Divers, is overhung by a down-growth from the alisphenoidal wings of the parasphenoid. This, in the Grebes, by its fusion with the free edge of the basitemporal plate, forms a pair of closed tubes opening on either side of the skull, behind the quadrate, and below the squamosal prominence into the aperture which serves also as the mouth of the tympanic cavity. The inferior and posterior of these two runs transversely across the skull, and forms the Eustachian tubes of the right and left sides of the head. The connection with the choanae is by means of a single median aperture immediately under the rostrum. The anterior runs forward as a pneumatic cavity into the body of the parasphenoid to terminate beneath the level of the foramen opticum. In the Divers, the form of this aperture is tubular, recalling that of the Penguins, the anterior wall of the tube being continued outwards behind the squamosal, but in the Grebes the anterior wall is deficient. Mammillary processes are but feebly developed; in the Divers the paroccipital notch is wide and shallow, it can scarcely be said to exist in the Grebes. There is a precondylar fossa, or rather groove, in the larger species of both families. The parasphenoid rostrum, in both Grebes and Divers, is somewhat inflated at the base, owing to the presence of the pneumatic cavity already described. The Lateral Aspect of the Cranium (Plate LXXII. figs. 3-6).- The tympanic cavity has a sharply defined aperture in the Divers (Colymbi), by reason of the considerable lateral development of the alisphenoidal wing of the parasphenoid. Within its mouth can be seen, distad, two large apertures, lying immediately behind the alisphenoidal wing just referred to : the upper is the pneumatic aperture of the parasphenoid, the lower is the Eustachian aperture ; caudad, and separated by a broad column of bone, lie the fenestra ovalis and the fenestra rotunda. The temporalis recess1, so well developed in the Steganopodes and Petrels, and to a lesser degree in the Penguins, is here represented only in the Divers, by a moderately deep fossa ; in the Grebes it is wanting. The posterior pneumatic cavity opening downwards, behind and above the fenestra ovalis, so well developed in the skull of the Tubinares, is wanting in both Grebes and Divers. 1 In my recent paper on the Osteology of the Penguins the temporalis recess was described as "leading eventually, in the dried skull, into the cranial cavity." This is quite a mistake. The correct interpretation of this is as follows :-In many skulls, e. g. Puffinus, above the trigeminal there lies a second foramen for the sinus transversus of the vena cephalica posterior-at times this is confluent with that for the trigeminal, e.g. Divers-and both these lie immediately outside, below and mesiad of the mouth of the recess in question. In the Penguin the foramen supplementary to the trigeminal lies immediately within the mouth of the recess, piercing its inner wall; owing to imperfect ossification, the mouth of the foramen in the dried skin extends upwards nearly the whole length of the recess. PROC. ZOOL. Soc-1899, No. LXVI. 66 |
