560 H. H. SWINMKUTON. 



Thus it appears tliat whatever may be thought coucerning 

 Amphibia and the Dipnoi, tliere is a general consensus of 

 opiuion tliat the Teleosts and Ganoids are not so closely allied 

 to Selachii as was once thought to be the case. In oi^der to 

 ascertain how far this is supported by the head skeleton, I pro- 

 pose now to briefly review what is known of this part in the 

 development and the adult of Teleostomes under the follow- 

 ing headings : — 'J'lie relation of the trabeculee to the para- 

 chordals ; the primordial cranium ; the relation of the 

 visceral skeleton to the cranium. 



The Relation of the Trabecul^e to the 

 Parachordals. 



In writing two interesting and instructive memoirs on the 

 development of the skull, Sewertzoff (97 and 99) has brought 

 together many scattered facts relating to this in the various 

 great classes of vertebrates, and has sought to reduce them to 

 some kind of order. Thus, for example, in dealing with the 

 position of the trabeculse, he has introduced the term "hori- 

 zontal" to describe their position when their hinder ends 

 fuse or arise in continuity with the extremity of the para- 

 chordals ; and " vertical," when fusion takes place with 

 their ventral surface and some distance behind their anterior 

 ends (99, p. 316). He also distinguishes two types of skull : 

 (1) that in which the trabeculas arise separately from the 

 parachordals ; (2) that in which the trabeculae arise in con- 

 tinuity with a plate which is or becomes the anterior part of 

 the parachordals. The fact that in the stickleback these 

 two types appear to be mere matters of individual variation 

 suggests that this distinction is not an important one. Again, 

 he tried to show that these two positions — " hoiizontal " and 

 "vertical" — were intimately associated with the time of 

 appearance of the mesocephalic flexure, of the medullary 

 flexure, and of the trabeculre. 



In the stickleback, as in Selachii, the time of appearance 

 for the trabeculae coincides with that of the greatest flexure. 

 Though the development of these parts takes place under 



