ORAL CIRRI OF SILUROIDS AND ORIGIN OP THE HEAD IN VERTEBRATES. 37 



able. In Teleostei it is almost equally great, but in Selachii there has 

 been much reduction and simplification. Vetter says of the adductor 

 mandibulse of Teleostei : " This muscular mass is nowhere found in the 

 form of the relatively simple undivided adductor of Selachii or of 

 Chimaera but always split into several portions. The Teleostei thus 

 stand in regard to the jaw muscles much nearer to Myxine than the 

 Selachii. 



Many great authorities have held that the adductor mandibulae is 

 the homologue of the adductores arcuum visceralium and that the jaws 

 represent a visceral arch, yet this view appears to me to be entirely 

 erroneus and to have turned subsequent investigations into a wrong 

 path. In spite of the admirable researches of Johannes Midler, 

 Ftirbringer and Vetter, much remains to be done, especially in the way 

 of comparison, since the muscles yield most conclusive evidence of the 

 correctness of the Cirrhostomial theory. 



The Cranium and other Parts. 



Concerning the skull it is not my intention to enter into any great 

 amount of detail, A cartilaginous tegmen cranii is not formed. 

 However across the great fontanelle runs the epiphysial bar, beneath 

 and slightly in front of which the pineal organ terminates. It divides 

 the fontanelle into an anterior and posterior portion and the anterior 

 part corresponds to the Selachian " Praefrontalliicke " (Gegenbaur) 

 inasmuch as a prolongation of the pineal organ is said to terminate at 

 the anterior border of the tegmen cranii. Parker has described the 

 tegmen cranii in the Salmon. An epiphysial bar is shown by Sagemehl 

 in Characinidae and Gyprinidae. It is represented by a rudimentary 

 block of cartilage, discovered by myself in Poh/pterus, and Gaupp has 

 followed its development in the tadpole, terming it the Taenia tecti 

 transversal is. The main fontanelle in the frog thus corresponds to 

 the Praefrontalliicke of Selachii. It is interesting to note that the 

 cartilage follows the wandering of the pineal organ backwards, but. the 

 dermal bones do not, the pineal organ shifting from between the 

 frontals to between the parietals. 



There is considerable variation in the floor of the brain, cartilages 

 remaining only as blocks or processes, paired or unpaired. 



A preorbital process is not formed in Trichomycterus, but by various 

 degrees it reaches a complete development as in Silarus. 



