92 



LECTURE V. 



logy of the supra-occipital with the supernumerary ' interparietal ' 

 bone of Mammals, beyond the anteriorly produced portion, vvliich, 

 however, is not developed from a separate centre in Fishes. 



When the skull is much compressed, the occipital spine is usually 

 very lofty, and in the Light-horse-man fish {Ephippus), expands 

 above its origin into a thick crest of bone, giving the skull the 

 appearance of a helmet ; but in low flattened skulls the spine is much 

 reduced, projecting merely backwards in the Pike and Salmon, and 

 being sometimes obsolete, as in the Remora ; in a few instances the 

 broad posterior part of the supra-occipital articulates with the neural 

 arch and spine of the atlas, and sometimes on the other hand, e. g. 

 in the Holibut, the entire bone is pushed by the par-occipitals upon 

 the upper surface of the skull, where it manifests the loss of symme- 

 try by the absence of the expanded plate on the left side of the spine, 

 which immediately articulates with the left parietal. 



The par-occipitals (par-apophyses of the occipital vertebra, Jig. 30 

 and 31.4, 4), are always wedged into the angles between the ex- and 

 supra-occipitals : they are of a sub-rhomboidal or conical form, with 

 the base towards the cranial cavity and the apex turned outwards and 

 backwards. The outer surface, in the Cod, is traversed obliquely by 

 a prominent ridge, ending at the lower and hinder projecting angle : 

 in the Cai'p the process is short, and comes from the middle of the 

 outer surface. 



In broad and depi'essed skulls the par-occipital forms a strong 

 crest, and exceeds the ex-occipital in size : in narrow and deep 

 skulls the proportions of these bones are commonly reversed, and 

 the par-occipitals sometimes disappear ; but in Ephippus, they are 

 as large as the ex-occipitals. In the Shad the par-occipitals unite 

 with the mastoids almost as in the Chelonia : and in the Poly- 

 pterus they become anchylosed to the ex-occipitals, as in Batrachian 

 Reptiles. 



In Synodus, Callichthys, and Heterobranchus, the par-occipital is 

 visible only at the back part, not at the ujiper part of the skull. 

 The inner surface of the par-occipital, like that of the ex-occipital, 

 is excavated for the lodgment of part of the posterior and exter- 

 nal semicircular canal of the enormous internal organ of hearing 

 in Fishes. The outer projecting process sujiports the upper fork of 

 the first piece of the scapular arch, sometimes, as in Ephippus, by a 

 distinct articular cavity. The parts of the occipital vertebra are 

 those which are commonly in Fishes the most completely ossified at 

 the expense of their primitive cartilaginous basis, and, in the Poly- 

 pterus, they become anchylosed into one piece, like the occipital 



