BY WILLIAM A. HASWELL, M.A., B.SC. 77 



of opposite sides are widely separated from one another by an 

 interval occupied by ligamentous fibres, but bearing teeth. The 

 two halves are more closely approximated in other Sharks ; in 

 some fCestracion, Scymnus) they are very intimately connected. 

 In the Rays the palatine and quadrate portions are usually not 

 well marked off from one another, and the two halves are 

 intimately united together in the middle line. In the Sharks a 

 palatine process is developed for articulation with the basis cranii, 

 but this is absent in the Rays. The articulation between the 

 palato-quadrate and lower jaw may be single, but, more usually, is 

 divisible into two parts. 



In most Sharks the two halves of the lower jaw, or Meckel's 

 cartilage, ai*e freely movable on one another, Cestracion being the 

 most important exception ; in the Rays the movability is much 

 less than in the Sharks, and the two rami may be united into 

 a rigid bar. 



In the simplest arrangement to be observed in the Elasmo- 

 branchii, the upper elements of the hyoid arch are similar in 

 function and in relation to the arch to those of the succeeding 

 branchial arches. They serve, that is to say, solely to suspend the 

 ventral portion of the arch, and the union with the side of the 

 cranium is slight and unimportant. In a further stage, such 

 as is represented in Cestracion, these elements become a little more 

 important and come into relation, slightly at first, with the palato- 

 quadrate cartilage and lower jaw, which they help to suspend, still, 

 however, being mainly related to the hyoicl ; in a yet more 

 advanced stage (most Sharks) the cartilage becomes thicker and 

 longer, its articulation with the skull becomes more complete, and 

 by its distal extremity, which develops a special mandibular 

 process, it is mainly related to the palato-quadrate and mandible, 

 the relation to the hyoid having now become a subsidiary one ; the 

 epi-hyal has now become a hyo-mandibular. In a further stage 

 (Rays) the hyoid undergoes a degeneration, loses its distinctness 

 from the branchial arches, and is attached to the base of the hyo- 

 mandibular, or is no longer directly related to it, but articulates 

 separately with the side wall of the cranium. Finally (as regards 



