THE CHONDROCRANIUM IN THE ICHTHYOPSIDA. 91 



ing the upper and under surfaces and the tip of the latter 

 exposed. 



The trabeculse {t) continue forwards in neaa-ly the same 

 horizontal plane as the main part of the notochord, the 

 apex of which, with the adjoining portions of the para- 

 chordals, is slightly depressed. The trabeculoe curve first 

 outwards to a point beyond their middle and then in, their 

 apices being about as far apart as their bases. They end 

 medially to the olfactory organs, a little behind the exter- 

 nal narial openings. The lower margin of each trabec- 

 ula is thickened, while above it is developed into a high 

 and thinner crest {trc) inclining slightly outwards. The 

 upper margin of this crest is extremely irregular and the 

 foramina for the optic and oculomotor nerves (o/" and oc) 

 are but partially enclosed. The crest gradually increases 

 in height from in front Ijackwards to the posterior end of 

 the trabecula where it terminates abruptly. 



The quadrates appear as two thin bands of cartilage 

 external to and at some distance from the parachordals. 

 They are concave anteriorly and their general direction is 

 downwards and outwards. In each quadrate can be al- 

 ready recognized three portions, a middle piece constitut- 

 ing a body (hq) from which arise an upper ascending 

 process {op) directed forward, inward and upward toward 

 the crest of the trabecula, and a lower descending process 

 {dp) ruiming outward, forward and downward to the 

 articulation with Meckel's cartilage. The body is some- 

 what lenticular while the processes are thin and more 

 laminate. 



Meckel's cartilage {ra) is already complete. The gen- 

 eral course of each ramus is obliquely inward and for- 

 ward until about the middle point where it curves 'still 

 more strongly inward to meet its fellow of the oi)posite 

 bide. At its base each ramus is stout but it tapers regu- 



