12 PROF, d'aect w. THOMPSON ON THE [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, StnUhio, and Aptenjx ; 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 rhe last of these three bojies : 

 in Parrots this chink is represented by the inner and outer pre- 

 cranial foramina of Mivart, a subdivision already incipient even 

 in the Eaven. Where we find the foramen in the Eatite, we in 

 most other cases find only a groove on the outer side of the pre- 

 frontal, shallow in the Eaven 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 prefrontal \ 



The lacertilian skull gives us no very close parallel to the very 

 remarkable suborbital arcade of the Psittacidaj, 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 Strinc/ojis, Ara, or 

 Clirysotis. And the junction between the two postorbital 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 consti- 

 tution 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 eccoccipital, while in regard to its anterior 

 and inferior portions we may assume that they are formed by 

 the hasitemporal of Parker.- Within, this tympanic chamber is 

 produced above into the sujoerior, below and behind into the 



^ In Huxley's ' Anatomj of Vertebrates,' where this bone is described, as usual, 

 as a laehrjmal, what is spoken of as the prefrontal is expressly defined as the 

 equivalent of the lateral mass of the ethmoid in Mammals, and the term is thus 

 used in a sense now entirely obsolete. 



