MORPHOLOGY OF TELEOSTRAN HEAD SKELETON. 561 



conditions which correspond to absence of cranial flexure, 

 yet in other Teleosts, e. g. Gobius^ which have a capacious 

 egg, and consequently flexed head during development^ the 

 horizontal position is assumed. 



The study of the stickleback's head (cp. fig. 57) shows 

 that flexure here is not so much at one point as at all points ; 

 that the anterior ends of the parachordals as well as the 

 closely associated trabeculfe are involved in this curvature; 

 and that the flexure here is much less than in the Selachian, 

 where the fore-brain lies practically under the hind brain, 

 and apparently does not involve the parachordals. These 

 facts lead to the inference that the position of the trabeculse 

 in Ichthyopsida is i-elated mainly to the degree of meso- 

 cephalic flexure and to the part which the parachordals take 

 in it. That is, so long as it is small — through not more than 

 90°, — there is nothing to prevent the trabeculte from uniting 

 with the ends of the parachordals; but immediately it over- 

 steps that, and the fore-brain begins to pass backwards under 

 the hind brain and parachordals, there will be a tendency to 

 carry the "anlage" of the trabeculaB backwards also, and 

 consequently the upper ends of these will either have to take 

 a, sharp bend forwards to unite with the extremities of the 

 parachordals, or have to fuse with their ventral surfaces. 



It was long ago pointed out by Balfour that '"'the cranial 

 flexure is least marked in Cyclostomata, Teleostei, Ganoidei, 

 and Amphibia, while it is very pronounced in Elasmobranchii, 

 lieptilia, Aves, and Mammalia" (81, p. 347). Consequently, 

 if the views just expressed contain any truth, the former 

 should exhibit the horizontal, and the latter the vertical 

 position for the trabecul^e. 



In his earlier paper Sewertzoff (97) recognised that in those 

 cases in which the mesocephalic flexure of the brain is least 

 marked the trabeculaj are approximately horizontal, and where 

 it is greatly marked they are vertical ; but he does not collate 

 these cases. He points out (99) that in both Petromyzon and 

 Acipenser the former position exists. From this and from 

 Parker's figures and descriptions of Lepidosteus ; from those 



