108 ANATOMY OF VERTEBRATES. 



from the frontal by tlie parietals, in the Sahnonoid, Clupeoid, 

 Murrenoid, and most ganoid fishes ; and is itself divided, in Amia 

 and Lepidosteiis, by a median suture ; these modifications tell 

 strongly against extending the homology of the superoccipital with 

 the supermimerary 'interparietal' bone of Mammals, beyond the 

 anteriorly produced interparietal portion ; which, however, is not 

 developed from a separate centre in Fishes. 



When the skull is much compressed the occipital spine is 

 usually very lofty, as in the Opah-fish and Argi/reiosus, fig. .38 : in 

 the Light-horseman fish {EpMppus) it 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 backward, as in the Pike and Salmon, and being some- 

 times obsolete, as in the Remora. In a few instances, the broad 

 posterior part of the superoccipital articulates with the neural 

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

 in the Halibut, the entire bone is jiushed by the paroccipitals 

 upon the ujiper surface of the skull, where it manifests the loss of 

 S3'mmetry by the absence of the expanded plate on the left side of 

 the spine. 



In broad and depressed skulls the paroccipital,' fig. 76, 4, forms 

 a strong crest, and exceeds the exoccipital in size ; in narrow and 

 deep skulls the proportions of these bones are commonly reversed, 

 and the parocciDitals sometimes disappear. In the Shad, the 

 paroccipitals unite with the mastoids almost as in the Chelonia ; 

 and in Pohjprion they are connate with the exoccipitals as in 

 Imtrachian and crocodilian Reptiles. In St/nodus, CalUchthi/s, 

 and Iletcrohranchus , the paroccipital is visil)le only at the back 

 part, not at the upper part, of the skull. The inner surface of 

 the paroccipital, like that of the exoccipital, is excavated for the 

 lodgment of part of the posterior and external semicircular canal 

 of the enormous internal organ of hearing in Fishes. The outer 

 projecting process supports the upper fork of the first piece of 

 the scapular arch ; sometimes, as in JEphlppus, by a distinct arti- 

 cular cavity. The neural parts of the occipital vcrtclira are those 

 which arc commonly in Fishes the most comjiletely ossified at the 

 expense of their primitive cartilaginous bases ; and, in Pohjpterus, 

 they become anchyloscd into one piece, like the occipital bone 

 of Anthropotomy, the superoccipital being as little developed as 

 in Protoptcnis. 



' The paroccipitals are not to bo confounded with tlic dermal bone called ' epiotic ' 

 by Professor Huxley, in his reproduction of Mailer's figure of the head of Pohipterus, 

 in the Government Publication, (clxviii.) p. 22, fig. 16. 



