20 



Fig. 3. 



Pterotic Region ^ 



^'H. M. 



Op. c' 



Pterygo-vom. 

 Lig. 



Mck. 



Qu. 



Ct. hy. 



stylohyal is attached. At the most ventral point the Meckelian car- 

 tilage has its articulation while inwardly and forwards the quadrate 

 is prolonged into the pterygoid process which in turn is connected 

 with the vomer by a ligament. Thus hyomandibular, opercular carti- 

 lage, symplectic, quadrate and pterygoid are at this stage in one 

 piece. 



Subsequently the cartilage becomes broken up, portions being re- 

 placed by bones which vary in extent in different fish. In Auchenapsis 

 for example a hyomandibular cartilage is left supporting the operculum 

 and a quadrato-pterygoid. In Trichomycterus at a really younger stage 

 the cartilage is broken up into hyomandibular and quadrate and a small 

 pterygoid cartilage is separately formed as also in Ictalurus (Ryder) 

 and many Urodeles (Wiedersheim and others). The hyomandibular 

 after reaching the bony stage may in some cases fuse with the skull 

 e. g. in Diodon (Stannius). 



