22 



and are botli attached to anterior projections from the 

 prefrontals in the manner described above. 



The suborbital and supratemporal chains of lateral 

 line ossicles are described with their respective sensory 

 canals. 



2. — The Palato-Pterygoid Arcade (Fig. 5). 



Ocular Side. 



Hyomandibular {Hm.). — This bone articulates with 

 the skull in two ways. First by a well marked ball and 

 socket joint situated at the anterior extremity of its 

 articular surface, and second by a long and less con- 

 spicuous facet behind this. The socket is a deep depres- 

 sion very obvious in dried skulls and situated between the 

 sphenotic and prootic. From this depression the other 

 facet passes upwards and backwards, and is placed mostly 

 if not entirely on the pterotic. The head of the hyoman- 

 dibular is cartilaginous in three places, at the two facets 

 for the skull and at the projection articulating with the 

 operculum. Parallel with the posterior edge, and at a 

 short distance from it, is a stout bony ridge (the most 

 strongly calcified part of the bone) which is closely 

 attached to the pre-operculum. In front of this a shaft 

 of cartilage passes downwards and forwards, which bears 

 a thin cartilaginous cap, and articulates with the inter- 

 hyal in such a way as to admirably illustrate what is now 

 a commonplace of vertebrate morphology, that the 

 hyomandibular is the modified dorsal segment of the hyoid 

 arch, which has in many forms lost its connection with 

 the hyoid arch, and has acquired on the one hand a con- 

 nection with the auditory capsule and on the other with 

 the jaw suspensorial apparatus. The Plaice is therefore 

 hyostylic. The remainder of the ventral edge of the 



