408 



On examining the region in which these bones are eventually 

 found, in an embryonic fish, I discovered, in their place, a delicate 

 inverted cartilaginous arch, attached anteriorly, by a very slender 

 pedicle, to the angles of the ethmoidal cartilage, and posteriorly con- 

 nected by a much thicker cms with the anterior portion of that 

 part of the cranial wall which encloses the auditory organ (6g. 8). 



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 

 distinct and separate portions the anterior division V-shaped, having 

 its anterior crus fixed and its posterior crus free above ; the posterior, 

 styliform, parallel with the posterior leg of the V and free below. 

 The anterior division supports the mandibular cartilage, the pos- 

 terior the hyoidean cornu. 



As ossification takes place, that part of the anterior crus of the 

 V-shaped cartilage which is attached to the ethmo-presphenoidal car- 

 tilage becomes the palatine ; its angle becomes the jugal ; between 

 these two the transverse and pterygoidien (represented by only one 

 bone in Gasterosteus) are developed in and around the anterior 

 crus : the tympanal arises in the same way around the free end of 

 the posterior crus. Thus these bones constitute an assemblage 

 which is at first quite distinct from the other elements of the sus- 

 pensorium, and immediately supports the mandibular cartilage. 



