572 



part of that bone, in the bony ganoids, but it is elsewhere represented 

 both in Amia and Lepidosteus, In Amia it has fused with the symplectic 

 to form a process of that bone which gives a supplementary articula- 

 tion to the mandible; while in Lepidosteus it is the independent, so- 

 called preoperculum of Parker's descriptions (interoperculum, Col- 

 linge), which, in that fish, has an independent articulation with the 

 quadrate. In the Muraenidae, the process and the symplectic seem 

 to both be indistinguishably fused with the body of the quadrate. 



The so-called quadrates of the osseous fishes are thus not all 

 equivalent structures. The articulations of the mandible with the 

 suspensorial apparatus are also not all similar; Amia being the one 

 known exception to the otherwise general rule, but Lepidosteus some- 

 what resembling it. A further development of the conditions found 

 in Amia might transfer the mandibular _ articulation from the palato- 

 quadrate to the hyomandibular. 



13. The side walls and floor of the skull of osseous fishes are 

 more or less completely double in the sphenoid (orbito-temporal, Gaupp) 

 and labyrinth regions; these walls being there represented in varying 

 proportions by membrane, cartilage or bone. Between the two walls, 

 in the sphenoid region, lies the myodome with its upper lateral or 

 trigemino-facialis chambers, while between the two walls in the labyrinth 

 region lie the membranous ears. The myodome and its trigemino- 

 facialis chambers are thus all intramural spaces. The floor of the 

 myodome is perforated by the hypophysial fenestra, while its roof is 

 perforated by the pituitary opening, and these two perforations of the 

 cranial floor are doubtless strictly homologous in all vertebrates; but 

 it must be determined, in each case, which one of the two perforations 

 is in question. The myodome lodges a cross-commissure of the pi- 

 tuitary veins, and is the probable homologue of the cavernous and 

 intercavernous sinuses of the human skull. The postpituitary portion 

 of its roof apparently always chondrifies, and is the postclinoid wall. 

 The prepituitary portion of its roof does not usually chondrify (Argyro- 

 pelecus may be an exception), and it and the basisphenoid, one or 

 both, represent the anterior clinoid wall. The orbital opening of the 

 myodome, on either side, is the sphenoidal fissure. 



The trigemino-facialis chamber of either side is, in the mail- 

 cheeked fishes, and probably in most teleosts, separated from the 

 myodome by a partition of bone. It lodges the trigeminus and related 

 sympathetic ganglia, and is the homologue of the cavum Mechelii of 

 the human skull. Its outer wall is, in all the mail-cheeked fishes 

 examined, excepting Cottus, represented by a narrow bridge of bone; 

 this wall of the chamber here forming the outer surface of the skull. 



