428 



Royal Society ; 



the ethmoidal cartilage, and posteriorly connected by a much thicker 

 crus with the anterior portion of that part of the cranial wall which 

 encloses the auditory organ (fig. 5). 



Fig. 5. — 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. Trabeculae 

 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 jom<r. Premaxilla. mx. Maxilla, d. Dentale. an. Angulare. o^. Arti- 

 culare. M^. Meckel's cartilage. 



The crown of the inverted arch exhibits an articular condyle for 

 the cartilaginous rudiment of the mandible. The posterior crus is 

 not, as it appears at first, a single continuous mass, but is composed 

 of two perfectly distinct pieces of cartilage applied together by their 

 edges. The anterior of these juxtaposed pieces is continuous below 

 with the condyle-bearing crown of the arch, and with its anterior 

 crus or pedicle (P.Q.) ; it is inclined backwards and upwards, and 

 terminates close to the base of the skull in a free pointed extremity. 



The posterior piece (S.Y. H.M.), on the other hand, has its broad 

 and narrow ends turned in the opposite direction. Distally, or below, 

 it is a slender cylindrical rod terminating in a rounded free extremity 

 behind, but close to, the condyle for the mandible ; above, it gradually 

 widens and becomes connected with the cranial walls. On its posterior 

 edge there is a convexity which articulates with the rudimentary 

 operculum, and below this it gives off a short styloid process, to 

 which the cartilaginous cornu of the hyoid is articulated. Thus 

 the cartilaginous arch, which stretches from the auditory capsule to 

 the ethmo-presphenoidal cartilage, consists, in reality, of two perfectly 



