ANATOMY OF VEUTEBRATES. 99 



these qualities seem to have been required in consequence of the 

 presence of a complex and largely developed diverging appendage, 

 which forms the framework of the principal flap or door, called 

 ' operculum,' figs. 81, 84, 34-37, that opens and closes the branchial 

 fissures on each side. The appendage in question consists of foru- 

 bones ; the one articulated to the tympanic pedicle is called ' pre- 

 opcrcular,' ib. 34 ; the other three are, counting downward, tlie 

 ' opercular,' ib. 35 ; the ' subopercular,' ib. 36 ; the interopercular,' 

 lb. 37. The hremapophysis is subdivided into two, three, or more 

 pieces, in different fishes, suturally interlocked together ; the most 

 common division is into two subequal parts, one presenting the 

 concavo-convex joint to the pleurapophysis, and called ' articular,' 

 ib. 29 ; the other, bifurcated behind to receive the point of 29, and 

 joining its fellow at tlie ojiposite end, to complete the Ixremal arch : 

 it supports a number of the hard bodies called ' teeth,' and hence 

 it has been termed the ' dentary,' il). 32. In the Cod there is a 

 small separate bone, below the j(_)int (jf the articular, forming an 

 angle there, and called the ' angular piece,' fig. 75, 30. 



In consequence of this extreme modification, in relation to the 

 offices of seizing and acting upon the food, the pair of hrema- 

 jjophyses of the present segment of the skull have received the 

 name of 'lower jaw,' or 'mandi1)le' {mandihula). The ha}mal 

 arch is, hence, called ' mandiluilar : ' the neural arch ' prosen- 

 cephalic : ' the entire segment is called the ' frontal vertebra.' 



TIic first segment, forming the anterior extremity of the neuro- 

 skeleton, like most 2:ieripheral jiarts, is that which has undergone 

 the most extreme modifications. The 

 obvious arrangement, nevertheless, of its 

 constituent bones, wlien viewed from be- 

 hind, after its detachment from the second 

 segment, affords one of the most conclu- 

 sive proofs of the principle of adlierence 

 to common type which governs all the 

 segments of the neuroskeleton, whatever 

 offices they may be modified to fulfil. Disaiticuintcd riimeucepbaiio .-uxii. 

 The neural arch, fig. 80, is pJamly mani- 

 fested, but is now reduced to its essential elements — viz., the 

 centrum, the neurapophyses, and the neural spine. The centrum 

 is expanded anteriorly, where it usually supports some teeth on 

 its under surface in fishes; it is called the 'vomer,' il). i3. The 

 neurapopliyses are notched (in the Cod), or perforated (in the 

 Sword-fish), by the crura or prolongations of the brain, which 

 expand into its anterior division, called rliinencephalon, or 



H 2 



