862 Vertebrata. 



no sternum and the ribs do not meet ventrally. The dorsal and anal 

 fins of Selachians are each supported by a laminate skeleton extend- 

 ingj on the one hand, into the base of the fin, and on the other, 

 between lateral myotomes of the trunk and tail. Each consists of a 

 number of cartilaginous pieces arranged like the skeleton of the 

 pectoral and pelvic fins. In Ganoids, Dipnoans, and Teleosteans, it 

 is replaced by the interspinous processes, usually dagger-like 

 bones, lying between the muscle plates, and united with the neural 

 spines ; or, on the ventral side of the tail, with the hsemal spines 

 by connective tissue : the interspinous bones which do not extend into 

 the fin bear each a single movably articulated ray. A pair of short 

 bones is usually intercalated between the ray and the interspinal. 

 The caudal fin is attached directly to the upper and lower arches 

 which are partly separated from the vertebrae at the hinder end of 

 the tail. 



The skull in Cyclostomes and Selachians is entirely carti- 

 laginous, but in the latter is frequently calcified on the surface ; 

 in the cartilaginous Ganoids also there is a similar thick-walled 

 cartilaginous capsule, but here it is partly covered with membrane 

 bones; in bony Ganoidei, Dipnoi and Teleostei, it consists originally 

 of cartilage, which is later not merely covered by membrane bones 

 but is also partly ossified, i.e., replaced by bone ; although some, 

 often a considerable portion, of the cartilage is retained throughout 

 life. The base of the skull, where it comes in contact with the spinal 

 column, is usually hollowed like a vertebra ; and on either side of 

 the foramen magnum, there is often an articular surface, which 

 corresponds with a similar one on the first vertebra. The eye lies in 

 a lateral cavity protected above by a roof-like projection of the skull, 

 which, in most Teleostei, is compressed between the eyes to a thin 

 cartilaginous or membranous partition, with the cranial cavity 

 prolonged as a narrow canal for the olfactory lobes above it. There 

 is a pair of smaller cavities anteriorly for the olfactory organs. 



In the Teleostei, aud in the bony Ganoids as a whole, the skull consists of a 

 larger or smaller amount of cartilage and a number of separate bones. The 

 cartilage bones (formed by ossification of the cartilage) ai-e : four occipitals 

 (basi-, supra-, and two ex-occipital s); of which all four, or the first and 

 the last two alone, bound the foramen magnum ; in the region of the labyrinth 

 the most important is the petrosal or prootic, there are in addition 

 the e p i o t i c and opisthotic; in the basal and lateral regions in front of 

 the parts just mentioned are the sphenoids (ali-, orbito-, and basi- 

 sphenoid); dorsal ossifications anterior and posterior to the orbit of each 

 side, the pre- and postfrontals; one or two ossifications at the anterior 

 end of the cartilaginous skull, the ethmoids. The following are membrane 

 bones: dorsal and anterior, a pair of nasals; then a pair of frontals (some- 

 times united, e.g., in the Ood) behind these again a pair of parietal s, lateral to 

 which on each side is a squamosal; ventrally a long flat unpaired bone 

 which covers the greater part of the floor of the skull, the parasphenoid; 

 and anterior to this the similarly azygos vomer. Besides these, others 



