MORPHOLOGY OF TELEOSTEAN HEAD SKELETON. 537 



The hyomandibular and symplectic {hym., sym.) are quite 

 continuous, the only indication of boundary between them 

 being, as before, the point of attachment of the stylohyal. 

 Owing to the gi'eat elongation of the symplectic, a feature 

 Avhich is still more strikingly seen in Syngnathus (McMurrich, 

 83, and Pouchet, 78) and Siphonostoma (fig. 58, sym.), this 

 point is no longer in the middle of the tract of cartilage 

 formed by these two elements. The axis of this tract is now 

 gently curved downwards and forwards from the auditory 

 capsule, and in association with this the line of connection 

 between the hyomandibular and the capsule has become 

 almost horizontal. The hyomandibular as a whole is tri- 

 angular, with the foramen for the hyomandibular nerve close 

 to its front side. 



The mandibular arch as a whole has essentially the same 

 shape as in Stage I, but all its parts are now well defined, and 

 consist of hyaline cartilage. 



The quadrate is triangular, with the mandibular articula- 

 tion at its apex. Resting as it does on the symplectic, it 

 seems to have been carried forward by the elongation 

 of this, so that the articulation now lies under the anterior 

 border of the orbit, and thus shows a striking contrast to the 

 salmon (Parker, 73, pi. iv, fig. 1) or trout (Winslow, 98, 

 pi. iv, fig. 28) ; in the former it lies beneath and in the latter 

 behind the orbit. Consequently in these forms the mouth is 

 half the length of the whole skull, and the symplectic and 

 hyomandibular have preserved the same proportions as in the 

 stickleback at Stage I. 



The metapterygoid process, when compared with the same 

 region of the salmon or trout (Text fig. 4, p. 571), is reduced 

 almost to vanishing point. A similar condition occurs in 

 Gobius and Atherina (Pouchet, 78) and Belone (Text fig. 2). 

 The vanishing point is reached in Syngnathus (McMurrich 

 and Pouchet) and. Siphonostoma (PI. 31, fig. 48), in 

 which this process is represented by the posterior upper 

 corner [qu.m.). This position is of course quite secondary, 

 for in the earliest stages the quadrate cartilage lies parallel 



