THE SKULL OF OSSEOUS FISHES. 113 



' ramus ' is applied in anthropotomy to each half of the mandible, 

 and each ramus consists of two, three, or more pieces in different 

 fishes. Most commonly it consists of two pieces, one (ha^mapophysis 

 pi'oper), articulated to the suspensory pedicle, and edentulous, ana- 

 logous to the maxillary ; and the other (hajmal spine) completing the 

 arch, and commonly supi)orting teeth, like tlie premaxillary. In the 

 Cod, and some otiier fishes, a third small piece is superadded, at the 

 angle of the posterior piece. That {ib. 29) which forms the sig- 

 moid concavity adapted to the tympanic trochlea is termed the ' ar- 

 ticular piece;' it sends upwards a pointed coronoid process, to which 

 the ligament from the maxillary bone, and the masticatory muscles 

 are attached ; one short square plate downwards, to join with the 

 angular {ib. 3o) ; and a long pointed process forwards, to be sheathed 

 in the deep notch of the anterior piece. This (32) is characterised 

 by the teeth, which, when present in the lower jaw, are always 

 supported by it ; whence its name of the ' dentary piece.' The 

 dentary is always deeply excavated, and receives a cylindrical car- 

 tilage* from the inner side of the hypo-tympanic, and the vessels 

 and nerves of the teeth. The great Sudis {jftg. 38.) and the Poly- 

 pterus have the splint-like plate ;J8 

 along the inner surface of the 

 ramus, answering to that which 

 Camper and Cuvier have un- 

 fortunately called 'operculaire,' 

 in the mandible of Reptiles, but to which I have given the name of 

 ^ spleniaV to prevent the confusion from the synonymy with the true 

 opercular bones of Fishes : and in both Sudis and Lepidosteus there 

 is superadded a small bony piece (ib. 29 a), answering to that which 

 Cuvier calls the ' sur-angulaire ' in Reptiles. These modifications, co- 

 existing with the true opercular bones, demonstrate the fallacy of the 

 idea that those bones are much developed homologucs of the posterior 

 pieces of the lower jaw of Reptiles ; an idea which could never have 

 been entertained by its propounders, had they appreciated the ge- 

 neral homology of the opercular pieces, as the diverging appendage 

 of the htemal arch of a cranial vertebra. 



The Diverging Appendage of the tympauo-mandibular arch con- 

 sists of the bones which support the gill-cover, a kind of short 

 and broad fin, the movements of which regulate the passage of 

 the currents through the branchial cavity, opening and closing the 

 branchial aperture on each side of the head. The first of these oper- 

 cular bones, which forms the chief medium of the attachment of the 



* M. Agassiz observes that this cartilage is the " reste de rancicn arc embryonal, 

 autour duquel les pieces osseuses se sont developpees." (xxii. i. p. 138.) 



VOL. IT. I 



Lower jaw {Arapahna gigna). 



