MORPHOLOGY OF TELEOSTKAN HEAD SKELETON. 559 



the absence of a cartilaginous wall between the auditory cap- 

 sule and cavum cranii by assuming a tendency for fenestra- 

 tion to proceed from the peripheries of nerve foramina 

 (p. 207), and considei-s that many of the parts point to a type 

 most nearly approached by the Notidanidfe (p. 227). 



Meanwhile, Huxley (76), by a comparative study of the 

 heads of Petromyzon and tadpole, was led to conclude that 

 there was '^ no sufficient foundation in the present state of 

 knowledge for regarding the Marsipobranch skull as one 

 which departs in any important respect from the general 

 vertebrate type " (p. 427). 



Since then various workers, especially during the last 

 decade, have brought forward much further evidence to show 

 that the same may be said of the other organs, and that 

 living Selachii are more specialised than Gegenbaur supposed. 

 Foremost among these was Beard (90), who, basing his 

 remarks chiefly on the structure of the brain, pronephros, 

 lung, and swim-bladder, proposed to place the Marsipobranchs, 

 Ganoids, and Teleosts in a group separate from and equal to 

 one containing the Selachii, Dipnoi, and Amphibia. Allow- 

 ing for a transference of the Dipnoi from the latter, Howes 

 (91) was led to a similar conclusion by a study of the urino- 

 genital organs. Klautsch's work on the development of the 

 vertebral column resulted in an entire agreement with Beard. 

 The most recent contributions to this subject are those of 

 Hatta (00) and Goette (01). The former, dealing with the 

 development of the pronephros of Petromyzon, says, ''The 

 whole system of the pronephros of Cyclostomata, Teleostei, 

 and Amphibia is not perfectly homologous with the Selachian 

 pronephric system" (p. 407). The latter, working at the 

 development of the gills and branchial vessels in fishes, finds 

 that the gills in Cyclostomes (Euterobranchies) are not the 

 homologues of those in Teleostomes and Selachii (Dermato- 

 branchies), but represent a very old type ; and that as regards 

 the origin of their afferent and efferent branchial vessels the 

 Teleostomes and Selachii exhibit two divergent lines of 

 modification. 



