ORAL CIEEI OF SILUROIDS AND ORIGIN OF THE HEAD IN VERTEBRATES. 9 



The Meckelian cartilage (Mch.) passes horizontally forward from its 

 articulation with the quadrate. An angular process, however, passes 

 downwards and backwards from the articulation. At some little 

 distance forward, the coronoid process is given off. The coronoid 

 process (Proc. cor.) projects vertically upward and bears the procartila- 

 ginous, cylindrical, horizontally placed coronoid piece (Cor. p.), which 

 reaches forward to the level of the prepalatine piece, its end, in fact, 

 approaching the maxillary tentacle. 



Directly below the coronoid process lies the proximal portion of 

 the submandibular tentacle (Subm. t.), and, some little distance in 

 front of this, is seen the corresponding part of the mental tentacle 

 {Ment t.). Beyond the level of the mental tentacle, the Meckelian 

 cartilage becomes distinctly more slender (m. Mch.) passing over con- 

 tinuously, in the symphysial region, into the corresponding portion of 

 the opposite side. Viewed from the front, the model reveals a most 

 remarkable conformation of the edge of the rostral portion and the 

 Meckelian cartilage, a bay being formed in the rostrum above, 

 and in the Meckelian cartilage below. A nodule (?n. v. s) attached 

 to the rostral region, in the bay, represents a tentacle-like pro- 

 cartilage support of the upper "Velum," or flap used to close the 

 mouth in respiration, and a corresponding nodule (m. v. s) on the 

 Meckelian cartilage represents the lower tentacle-like support of the 

 lower velum. 



The stylohyal bears the enormous ceratohyal (Hi/.) which is much 

 ■expanded at its anterior and posterior ends. 



The middle portion which is strengthened by perichondrial bone 

 is thinner. Anteriorly are found the smaller hypohyals. 



Model of Trichomycterus (Figs. 5, 9). 



Specimen about 2"5 cm in length. Sections 20 p. thick. Every second 

 section drawn with magnification 18 (Zeiss Oc. 14 Obj. aa.). Height 

 above table of drawing 19 cm. Thickness of wax plates T6 mm. 

 Model about 13*5 cm long by 185 broad. Reconstructed so far as 

 to show the hyomandibular. 



The head was about the size of a small pea. The replacement 

 of cartilage by bone has gone on in places so far as to seriously 

 interfere with the modelling although the specimen was very young, 



