| OCR Text |
Show 12 PROF, D'ARCY W. THOMPSON ON T H E [Jan. 17, seen a case) that in certain of these Anserine birds the two unite in a suborbital ring. In some birds, but not in very many, this bone presents, in its anterior wall, a conspicuous foramen, which is especially well seen, for instance, in Rhea, Struthio, and Apteryx ; but it must not at all be confused, as in certain birds it might possibly be apt to be, with the chink formed between prefrontal, frontal, and ethmoid in those birds where the first meets with the last of these three bones : in Parrots this chink is represented by the inner and outer pre-cranial foramina of Mivart, a subdivision already incipient even in the Raven. Where we find the foramen in the Ratite, we in most other cases find only a groove on the outer side of the prefrontal, shallow in the Raven and the Parrot, deep in many Passerines, e. g. Acridotheres, in Dacelo, in the Herons, very deep in the Penguins, the Eagles, Vultures, &c. It seems to me more than probable that where we have this foramen developed its outer wall is contributed by a true lachrymal, precisely as the similar foramen is bounded by the prefrontal and lachrymal in Iguana; but that in the other cases we have good grounds for abandoning the term lachrymal, and accepting the bone in question as a true prefrontal1. The lacertilian skull gives us no very close parallel to the very remarkable suborbital arcade of the Psittacidae, but we may trace in it an indication of the latter's constituent parts and probable method of formation. The postfrontal runs in the Lizards, perhaps still more in Hatteria, a long way down the inner and anterior side of the superior or ascending ramus of the jugal, that ramus which in birds is aborted, as is the posterior one in the Lacertilia. It is but crossing a very little gap for the prefrontal and postfrontal to join below, and, separating from contact with maxillary and jugal, to form such a suborbital bar as we find in Stringops, Ara, or Chrysotis. A nd the junction between the two post orbital processes, that is to say the postfrontal and squamosal processes, that we find in the Cockatoos, will be seen simply to enclose a supratemporal arcade, bounded by the same bones and occupied by the same (temporal) muscle as it is in its vastly greater development in the Lacertilia. The auditory meatus or tympanic orifice is surrounded by an imperfect ring of bone, irregular in outline, of whose real constitution we are again left somewhat in doubt. It seems plain that its upper border is contributed by the squamosal, possibly in part by the opisthotic, its outer or posterior border by the thin, shell-like extension of the exoccipital, while in regard to its anterior and inferior portions we may assume that they are formed by the basitemporal of Parker. Within, this tympanic chamber is produced above into the superior, below and behind into the 1 In Huxley's ' Anatomy of Vertebrates,' where this bone is described, as usual, as a lachrymal, what is spoken of as the prefrontal is expressly denned as the equivalent of the lateral mass of the ethmoid in Mammals, and the term is thus used in a sense now entirely obsolete. |
