The oral cirri of Siluroids and the origin of the head in Vertebrates. 387 



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

 cartilage becomes distinctly more slender [ni. Mck.) 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 {m.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 {Hy.) which is much 

 expanded at its anterior and posterior ends. 



The middle i)()rtion which is strengthened by perichondrial bone 

 is thinner. Anteriorly are found the smaller hypohyals. 



Model of Trichortiycterus (Figs. 5, 9). 



Specimen about 2,5 cm in length. Sections 20 f.i thick. Every 

 second section drawn with magnification 48, Zeiss Oc. 14, Obj. aa, Ht, 

 above table of drawing 19 cm. Thickness of wax plates 1,6 mm. 

 Model about 13,5 cm long by 18,5 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, 

 the nerves being almost in an embryonic condition. Apparently in 

 all the S. American Siluroids ossification begins at a remarkably 

 early stage. 



The auditory region shows much replacement of cartilage by bone, 

 only irregular and asymmetrical projections being left. Cartilage only 

 remains in fact in the neighbourhood of the hyomandibular articulation. 

 A pterotic ridge is little developed and the hyomandibular articulation, 

 which is very great in extent anteroposteriorly, may be said to be on 

 the pterotic ridge itself. The cartilaginous floor of the brain case is 

 only represented by three isolated l)locks, one unpaired and median 

 at the level of the auditory labyrinth, and a pair situated below the 

 foramen of exit of the F'acial and Trigeminus nerves. 



The supraorl)ital band is the only remaining cartilage of the 

 posterior part of the orbital wall, it is somewhat triangular in section. 



