558 H. H. SWINNERTON. 



ment of Siplionostoma the reduced premaxillary processes 

 end in similar tissue, wliicli, however, in the latter stages, 

 forms a separate cartilage homologous with Sagemehl's 

 rostrale (85, p. 100). It seems highly probable, therefore, 

 that the cartilage we are considering in the stickleback also 

 has the same homologies. Whether the same is true of the 

 similar layer on the mesethmoid bone is an open question. 

 Whatever their morphological significance may be, func- 

 tionally these cartilages serve as a means for allowing the 

 premaxillse to slide on the ethmoid. 



IV. General Considerations. 



Looking back over the past thirty years of investigation 

 into Teleostean phylogeny, one of the many features which 

 calls for attention is the gradual shifting of the centre of 

 interest from the Elasmobranchs to the Marsipobranchs. At 

 the beginning of this period Haeckel was so impressed 

 by the many peculiarities of the latter group that he created 

 for its reception a division, the Monorrhiua, equal in value 

 to another, the Amphirrhina, containing all the other craniata 

 (70, p. 507). Kepresenting his views of the inter-relationships 

 of the latter in a diagram, he placed the Selachii at the point 

 of origin of the rest (p. 513). In these views ho was followed 

 by Gegenbaur (72), who, whilst he acknowledged the existence 

 of several special features in the Selachii, considered that on 

 the whole the organisation of the ancestral form of other 

 fishes must have been similar to that presented by them (p. 22). 

 Referring especially to the cranium, he gave expression to 

 the opinion that the primordial cranium of higher vertebrates 

 was but a transitory reproduction of the cartilaginous shark's 

 skull, and that it was impossible for the latter to be derived 

 from an originally bony condition. 



The natural outcome of all this could only be an attempt 

 to harmonise the conditions exhibited by the Teleosts and 

 the closely allied Ganoids with those found in Selachii. Thus 

 Sagemehl (84), in dealing with the cranium of Amia, explains 



