. Elements of comparative anatomy. Anatomy, Comparative. SKULL OF VERTEBRATA. 453 wliicli passes obliquely backwards from the orbit, this bone forms a pillar between the canals of either side. Not unfrequently it seems to disappear altogether. On the ventral side of, and along the pri- mordial cranium there extends the large parasphenoid (Figs. 242, Ps; 243, 5), which has been already seen in the Sturiones. It is seldom that the whole of the primordial cranium is retained on the upper surface; as a rule, there are spaces in it, which are overlaid by covering bones. Nearest to the posterior pri
. Elements of comparative anatomy. Anatomy, Comparative. SKULL OF VERTEBRATA. 453 wliicli passes obliquely backwards from the orbit, this bone forms a pillar between the canals of either side. Not unfrequently it seems to disappear altogether. On the ventral side of, and along the pri- mordial cranium there extends the large parasphenoid (Figs. 242, Ps; 243, 5), which has been already seen in the Sturiones. It is seldom that the whole of the primordial cranium is retained on the upper surface; as a rule, there are spaces in it, which are overlaid by covering bones. Nearest to the posterior primary region there are the two parietals (Fig. 244, 7); these are sometimes sepa- rated from one another by an anterior process of the supra-occipital (3). In front of them there are the frontals, which are often re- placed by a united frontal (11). At the sides of this are the two post- frontals (12), which extend as far as the squa- mosal, and take part in the articulation of the hyomandibular. In the ethmoidal region there is a median piece, ethmo'idale medium (1(3), to the sides of which the lateral ethmoids are attached (14) (frontalia anteriora. Cuvier). The latter form the base of the nasal capsules. The median portion of the ethmoids is often per- manently cartilaginous. The vomer forms the covering piece of the base of the ethmoidal region; posteriorly this is united to the para- sphenoid ; it is paired in Lepidosteus. § Fig. 244. Skull of a Gad us from above. 3 Supra-occipital. 4Epi- otic. 6 Squamosal. 7 Parietal. 11 Frontal. 12 Postfrontal. 14 La- tcral ethmoid. 16 Me- dian ethmoid. The mandibular apparatus of the Selachii is not completely retained in the Gano'idei and Teleostei, owing to the presence of bony struc- tures in its place. It is complicated by the union of the hyomandibular with the bones formed from the palato-quadrate cartilage. The primitive relations, as they are seen in the embryonic condition, show that it has been derived from the arrange-
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectanatomycomparative
