274 MR. W. K. PARKER ON THE OSTEOLOGY OF BAL^NICEPS REX. 



reptiles, the central is the chief element. In Coluber natrix and in Lacerta agilis, Rathke 

 has observed the epiotic centres before they had coalesced with the main part of the 

 supra-occipital, and the mastoid element before it was fused to the ex-occipital (see 

 Huxley, Croon. Lect. p. 61). In most osseous fishes the three supra-occipital elements 

 continue distinct throughout life. 



According to Professor Goodsir, the interparietal of the mammals is part of another 

 sclerotome, being the homologue of the divided roof-bone of the temporal cincture, the 

 so-called ' parietals ' of oviparous vertebrata : Professor Goodsir calls these upper tem- 

 poral elements temporo-parietals. 



The mastoid would appear to be present in Colymbus cristatus (in very young birds), 

 from some observations made by Dr. Hallman (Croon. Lect. p. 54) ; but examination 

 of the development of Divers, Auks, Penguins, and allied birds is very desirable ; and 

 perhaps the Struthionidae, taken early enough, might reveal a distinct osseous centre for 

 this bone. 



We consider Cuvier's ' rocher ' in fishes and Professor Owen's 'petrosal' to be the 

 mastoid ; the large bone notched or perforated in front by the trigeminal nerve being, 

 as Professor Huxley has unanswerably shown, the true petrosal (Croon. Lect. p. 24). 



In the Common Mole, Talpa europaa, the mastoid is very large, and forms more of 

 the cranial wall than its serial homologue, the squamosal ; in this creature the petrosal 

 forms nearly the whole of the labyrinth. 



That the mastoid is not necessarily connected with the petrosal is shown in the skull 

 of the Common Bat, Vespertilio niurinus ; for in this instance (and apparently in the 

 Brazilian, Molossus ohscurus) , the mastoid is anchylosed to the ex- or lateral-occipital, 

 and is only connected by membrane to the squamosal and petrous bone, the latter 

 bone being totally distinct from all the surrounding elements. 



We now return to the occipital bone of adult birds. 



In the Hornbills {Buceros), where the mere size of the head (although extremely light) 

 causes the expenditure of much muscular force, a tubercle exists above the foramen 

 magnum, for the attachment of a very thick elastic ligament : this tubercle is wanting 

 in the Balseniceps and the Adjutant. 



In the Common Heron the occipital resembles that of the Balaeniceps, but it is higher 

 in proportion to its width ; its upper boundary is almost straight, there being only a 

 slight angle in the centre, where the transverse occipital crest is confluent with the 

 sagittal ; the par-occipital processes also are relatively much smaller. But the elegant 

 basi-cranial pterapophyses, and the open Eustachian groove between the proper basi- 

 sphenoid and the basi-temporal, are singularly like what we find in the Balaeniceps. 

 The Adjutant and the Albatros have this canal open in the dry skull, but in most 

 birds it is more or less covered in, leaving only a central aperture. 



In the Pelican the line of junction between the occiput and the upper part of the 

 skull forms a very obtuse angle, these surfaces being at nearly a right angle in the 



