A treatise on zoology . ; , neural arch ; , nerve-cord; , neuralcanal ; , neural spine ; nt, notochord ; , pleural rib. seem to be due to the genuine diphycercal character of the tail,and not, as Balfoin- supposed [30], to the loss of the caudal finitself. This character indicates a primitive structure (p. 101), andweighs against Dollos theory. The notochord extends to near the pituitary region ; there isno distinct joint between the skull and the vertebral column, Avhichpass gradually into each other. Several vestigial vertebral segmentsseem to be incorporated into the hind
A treatise on zoology . ; , neural arch ; , nerve-cord; , neuralcanal ; , neural spine ; nt, notochord ; , pleural rib. seem to be due to the genuine diphycercal character of the tail,and not, as Balfoin- supposed [30], to the loss of the caudal finitself. This character indicates a primitive structure (p. 101), andweighs against Dollos theory. The notochord extends to near the pituitary region ; there isno distinct joint between the skull and the vertebral column, Avhichpass gradually into each other. Several vestigial vertebral segmentsseem to be incorporated into the hind part of the skull behind thevagus nerve. In Ceratodus, besides the hypoglossal roots, twocomplete spinal nerves pass through this region; three neuralarches and spines are found attached to it above, and a pair ofenlarged cranial libs articulate to it at the sides (Figs. 20G, 209).Such cranial ribs are found in all living Dipnoi, and even in suciv SKELETON 235 early genera as the Devonian Scaumenacia. The bone described. by Huxley as an exoccipital, the only endochondral ossificationin the skull of living Dipnoi, appears to represent the first of the ^36 DIPNOI occipital neural arches, since it is placed l^etween the hypoglossaland the first occipitospinal nerve (Fiirbringer [lllft]). The chondrocranium, complete, thick, and without fontanellesin Ceratodus (Fig. 206), is less complete in the Dipneumones,
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
