The oral cirri of Siluroids «nd the origin of the head in Vertebrates. 399 



and size of the nasal capsules and the presence of rostral cartilages. 

 In embryos of various vertebrates it is due merely to the mode of 

 growth of the brain ami olfactory organs. 



In Silurus the premaxillary piece is represented by procartilage 

 above and between the premaxillary bones. In many Teleostei it becomes 

 a solid block of cartilage. It is mentioned by Stannius as a special 

 part of the snout in front of the nasal septum. "In Coitus and Belone^ 

 a small discrete cartilage, applied to the anterior end of the skull, is 

 covered by the premaxillae.'' "In Malthaea it forms a considerable 

 free projection on the skull" (Stannius). 



Sagemehl gave the rather unsatisfactory name "Rostrale" to this 

 block and mentioned its existence in Scomberesocidae, Cyprinodontidae, 

 Scopelidae, Cyprinidae, Anacanthini {Macrurus)^ Acantliopieridae and 

 Fleciognathi. "From its relations to the premaxillae we have every 

 ground for the supposition that it originally formed the basis (Grund- 

 lage) of these bones. The fact that it occurs in far removed forms 

 of Teleostei allows the conclusion that it is of great antiquity, and 

 thus we may expect to find it in lower fish." "It is found in most 

 distantly related groups, and this points to its being an inheritance 

 from a very remote ancestral form." 



Sagemehl goes on to compare it to a small cartilage between 

 the ends of the palato-quadrate in Eeptanchus. This view cannot 

 be correct. Since it is unpaired the piece cannot, according to Sage- 

 mehl, correspond to a labial of Selachii. A premaxillary block also 

 occurs in the Pharyngognathi (Labrus) and probably in many others 

 not yet investigated, especially where the premaxillae possess a verti- 

 cal upward projection, sliding on the ethmoid region. Among the 

 older anatomists this piece has apparently also received the name 

 prenasal cartilage, but I have not succeeded in running this term down. 



An interesting feature in connection with this block is that the 

 median velar support is shown by comparison to be a posterior pro- 

 longation from it serving to support the velum, or fold of respiratory 

 function, which lies behind the premaxillae. "In many Teleostei, 

 mucous folds, placed behind the jaws, hinder the outflow from the 

 mouth of the water which has been gulped in" (Stannius). The 

 median velar tentacle-like structure is shown in Silurus and Tricho- 

 mycterus (Figs. 3 and 5) and it occurs in many Teleostei. There 

 may be also a lower median tentacle-like support of a lower velum, 

 and these two give the appearance shown in Silurus (Fig, 3). It is 

 the mode of development of these structures in the embryo which 



26* 



