MOltPHOLOGY OP TRLEOSTEAN HEAD SKELETON. 519 



posterior insertion has come to lie under the hinder border 

 of the fossa (fig. S8,vie., o.pr.), Avhilst that on the eyeball is now 

 tending to pass down with the latter to the level of the basis 

 cranii. In consequence of this tendency, which finds its con- 

 summation in the adult (fig. 59, me.), the course of the muscles 

 is still more obliquely forwards and less obliquely upwards. 



Passing to the anterior or trabecular portiou of the chondro- 

 cranium, the ethmoid plate (fig. 9, e.) has greatly elongated. 

 The connections between the mesethmoid and the parethmoid 

 cornua have become more extended, and give the appearance, 

 when viewed dorsally (fig. 4), of a rudimentary tegmeu cranii. 

 In front of the mesethmoid the ethmoid still retains its original 

 plate-like character, but the pre-ethmoid cornua (e. j/r.) have 

 now become slightly diawni out dorso-veutrally. Fig. 30, which 

 represents a section through the pre-ethmoid region (e.), passes 

 through the extreme upper end of this process (e. pr.) ; a 

 few sections further back the two parts become continuous. 



Apart from its great length the ethmoid of Siphonostoma 

 (fig. 48, e.) is just the same as in the stickleback. 



Concerning the regions in which, in the stickleback, sup- 

 pression has taken place, remnants of the supra-orbital 

 bands are still seen in the backward pi'ocesses of the rudi- 

 mentai'y tegmen cranii ; whilst those parts of the trabeculre 

 which in the previous stage were fused together, now remain 

 as a posterior prolongation of the ethmoid plate. Of the 

 parts which have disappeared no trace remains; nor is any 

 cause forthcoming-, unless it be that tliey are functionally 

 replaced by the froutals and parasphenoid respectively. 



The epiphysial cartilage, now quite free of the ethmoid 

 region, has grown considerably posteriorly, and become 

 connected with the hinder cranial roof by a thin strip of 

 cartilage running through bone (e. p. s.). This strip is the 

 homolop'ue of the massive cartilao^e which lies between 

 the small lateral cranial fontanelles in the adult salmon, 

 and of the small process (fig. G2, e. p. s.) of the epi- 

 physial cartilage which projects back into the posterior 

 cranial foutanelle {p. /.) of the larva. The condition in 



