84 ANATOMY OF VERTEBRATES. 



Lepidoslren resembles the Plagiostomes. Two ridges rise from the 

 upper surface of the basioccipito-sphenoidal plate, near its outer 

 margin, and support the cartilaginous lateral walls of the cranium. 

 The cranial cavity is defended above by a longitudinal bony roof, 

 fig. 67, 11, nearly coextensive with the bony floor beneath: the 

 roof commences behind by the sj^ine or point which overhangs the 

 exoccipltals, gradually expands as it advances, resting upon the 

 cartilaginous walls of the cranium, is then suddenly contracted, and 

 is united anteriorly by fibrous ligament to the ascending process of 

 the palato-maxillary arch, 20, and to the base of the naso-premax- 

 illary plate, 15. A strong sharjj crest or sp)ine rises from above the 

 whole of the middle line of the cranial roof-bone, which may be 

 regarded as representing the mid-frontal, the parietal, and super- 

 occipital l)ones, or, in more general terms, the neural spines of the 

 three cranial vertebrse : but this supracranial bone not only covers 

 the medulla oblongata, cerebellum, optic lobes, pineal sac, and 

 cerebral hemispheres, but also the olfactory lobes. The lateral 

 cartilaginous walls of the cranium are continued forward from the 

 acoustic capsule between the basal and superior osseous plates: 

 the part perforated by the fifth pair of nerves, and jjrotectino- 

 the side of the optic lobes, represents the 'alisphenoid': the 

 next portion in advance, protecting the sides of the cerebral 

 hemispheres and perforated by the optic nerve, answers to the 

 orlntosphenoid : and the cartilage terminates by a ' prefrontal' part 

 which is perforated hj the olfactory nerve, and which aljuts laterally 

 against the ascending or palatine process of the maxillary arch. 



The extension of the lateral cartilages of the cranium forward 

 and downward to form the articulation for the lower jaw, is like 

 that in the Chimera and batrachian lar^'a, fig. 69a, e ; but ossifica- 

 tion has co-extended along two tracts, which con- 

 verge as they descend, one, fig. 41, 28, from above 

 and behind to the outer, the other, ilj. 2.3, from before 

 to the inner, side of the cartilaginous mandibular 

 Cranial spines and J*'™*'' wliicli thcse bouy platcs Strengthen and sup- 

 """'2:r» ''"''' port like the backs of a book. The posterior of these 

 IS the tympanic, the anterior one the pterygoid, 

 which is confluent with the palato-maxillary bone, the dentige'rous 

 part of which extends outward, downward," and backward, fi'g. 67, 

 21, but does not reach, as in the Sharks and Eays, the mandilxilar 

 joint. From the upper part of the palato-maxiilary a compressed 

 sharp process, ib. 20, ascends obliquely backward, and terminates 

 m a point : the inner side of this process is closely attaolied by 

 ligament to the fore and outer part of the frontal portion of the 



