426 Roijal Society : — 



of the squamosal is seen entering into its outer boundary above the 

 mastoid. On regarding the turtle's skull in the same way, there is 

 seen, occupying the same position, the bone which Cuvier, as I 

 venture to think, most unfortunately, named "mastoid." But if 

 the arguments brought forward above be, as I believe, with Hall- 

 mann, they are, irrefragable, this bone cannot be the mastoid ; and 

 I can discover no valid reason why it should not be regarded as 

 what its position and relations naturally suggest it to be — the squa- 

 mosal. Its connexions with the mastoid, petrosal, and quadratum 

 are essentially the same as those of the squamosal in the bird and the 

 mammal. The quadratum and articulare of the turtle are on all 

 hands admitted to be the homologues of the similarly-named bones 

 in the bird, and therefore all the reasonings which applied to the 

 one apply to the other. When the petrosal, mastoid, and squamosal 

 are determined in the turtle, they are determined in all the Reptilia. 

 But the CrocoJilia, Lacertilia, and Ophidia differ from the turtle and 

 CheJonia generally, in that their mastoid is, as in the bird, anchylosed 

 with the exoccii)ital. The squamosal, again, which in the Crocodilia 

 essentially resembles that of the turtle, becomes a slender and elon- 

 gated bone in the Lacertilia, and still more in the Ophidia, in 

 which the quadratum is carried at its extremity *. 



With respect to the skull of Fishes, the following extracts contain 

 the most important of the views put forth by Prof. Huxley. 



In discussing the structure of the skull of the Carp, Prof. Huxley 

 remarks : — When viewed from within, the foramen ovale is seen to 

 be, as in the bird, a mere conjugational foramen between the alisphe- 

 Hoid and the bone which follows it ; and on an external view, the 

 third division of the trigeminal is seen to pass entirely in front of the 

 last-named bone. 



The minutest scrutiny of the relations of this bone only strengthens 

 the conviction suggested by the first view of it, that it is the homo- 

 logue of the petrosal of birds, and therefore of mammals and rep- 

 tiles. As in the bird, the anterior margin of the fish's petrosal is 

 divided into a superior and an inferior portion, which meet at an 

 angle, the superior portion articulating with the parietal (and 

 squamosal), the inferior with the alisphenoid. Inferiorly the 

 jjctrosal articulates with the basisphenoid, and, to a small extent, 

 with the basioccipital. Posteriorly it articulates with a bone 

 through which the pneumogastric passes, and which, guided by 

 the analogy of most Reptilia, of Amphibia, and of birds, I believe 

 to represent the coalesced or connate mastoid and exoccipital. The 

 bone lodges the anterior part of the auditory labyrinth ; its middle 

 region corresponds with the middle of the mesencejihalon. But as 

 it does not separate the auditory organ from the cavity of the skull, 

 it naturally presents no foramina corresj)onding with those through 

 which the portio dura and portio mollis pass in Abranchiate Verte- 

 hrata and Amphibia. There is one relation of the petrosal in the 



* See for the manner in which this is brought about, Ratlil^e's ' Entwick. d. 

 Natter.' Rathke, it should l)e said, regards this bone as the tympanicum, but its 

 primitive place and mode of origin are those of the squamosal of the mammal. 



