84 ANATOMY OF VERTEBRATES. 



Lepidosiren 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 spine or point which overhangs the 

 exoccipitals, 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 sharp crest or spine 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 bones, or, in more general terms, the neural spines of the 

 three cranial vertebrae : 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 protecting 

 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 

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

 which is perforated by the olfactory nerve, and which abuts 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 Chimaera and batrachian larva, 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, ib. 23, from before 

 to the inner, side of the cartilaginous mandibular 

 joint, which these bony plates strengthen and sup- 



uvveT do%m Lepi ' P ort ^ e tne backs °f a book. The posterior of these 

 is the tympanic, the anterior one the pterygoid, 

 which is confluent with the palato-maxillary bone, the dentigerous 

 part of which extends outward, downward, and backward, fig. 67, 

 21, but does not reach, as in the Sharks and Rays, the mandibular 

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

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

 in a point : the inner side of this process is closely attached by 

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



