409 



The proximal end (H.M.) of the posterior styliform division gradu- 

 ally becomes articulated with the cranial walls, and, ossifying, is con- 

 verted into the temporal. The distal cylindrical end (S.Y.) becomes 

 surrounded by an osseous sheath, which at first leaves its distal end 

 unenclosed. The bone thus formed is the symplectique, which is at 

 first free, but eventually becomes enclosed within a sheath furnished 

 to it by thejugal, and so strengthens the union of the two divisions 

 of the arch already established by the junction of the tympanal with 

 the temporal. The symplectique and temporal do not meet, but leave 

 between them a cartilaginous space, whence the supporting pedicle 

 of the hyoid, which ossifies and becomes the osselet styloide, arises. 



The operculum, suboperculum, interoperculum, and preoper- 







Fig. 8. Cranium and face of young Gasterostei at different ages. The left-hand 

 figure is a view of the base of the skull of a very young fish. The middle figure 

 represents the under aspect, and the right-hand figure, a side view of a longitu- 

 dinal section, of a more advanced stickleback's skull. 



C. Notochord. P. Pituitary space. AC. Auditory capsules. T. Trabecula3 

 cranii. E.V. Ethmovomerine cartilage. P.Q. Palatoquadrate arch. Qu. Qua- 

 dratum. S.Y. or Sy. Symplectic. H. Hyoidean arc. H.M. Hyomandibular carti- 

 lage. The other letters have the same signification as in the preceding figures, 

 except pmx. Premaxilla. mx. Maxilla, d. Dentale. an. Angulare. at. Arti- 

 culare. M. Meckel's cartilage. 



culum are not developed from the primitive cartilaginous arch, but 

 make their appearance as osseous deposits in the branchiostegal mem- 

 brane, behind, and on the outer side of, the posterior crus. 



