ON THE VERTEBRATE SKELETON. 237 



vertebrata, and to which piece the term 'styloid' or 'stiliform' has been 

 given from the fish up to man (see Table I.). The homology, indeed, is so 

 obvious, that M. Agassiz, in accepting the conclusion of M. Vogt, that the 

 bone (fig. 5, 34), peculiar to osseous fishes, which so rarely articulates di- 

 rectly with the mastoid or with the hyoid arch, and so constantly sustains 

 the distal segment of the operculum, was the homologue of the 'processus 

 stiliformis ossis temporis,' nevertheless retains the name 'stylo'ide' for the 

 part no. 3s in question. 



The true homology of no. 34, already explained, removes the anomaly of 

 viewing that peculiarly piscine bone as the homologue of a constant element 

 of the hyoid arch in all the vertebrate classes, and the greater anomaly of 

 the introduction of a new element — a styloid piece of the os hyoides — in 

 addition to the 'styloid process of the temporal' in fishes. The 'stylohyal' 

 articulates below to the apex of a triangular piece (39), which is pretty con- 

 stant in fishes, and to which I give the name of ' epihyal,' as being the upper 

 of the two principal parts of the cornu or arch : the third longer and stronger 

 piece is the ' ceratohyal ' (ib. 40). 



The keystone or body of the inverted hyoid arch is formed by two small 

 subcubical bones on each side, the 'basihyals' (ib. 41). These complete the 

 bony arch in some fishes : in most others there is a median styliform ossicle, 

 extended forwards from the basi-hyal symphysis into the substance of the 

 tongue, called the ' glossohyal ' (ib. 42), or ' os linguale'; and another symme- 

 trical, but usually triangular, flattened bone, which expands vertically as it 

 extends backwards, in the middle line, from the basihyals ; this is the ' urohyal' 

 (ib. 43). It is connected with the symphysis of the coracoids, which closes below 

 the fourth of the cranial inverted arches, and it thus forms the isthmus which 

 separates below the two branchial apertures. In the conger the hyoidean 

 arch is simplified by the persistent ligamentous state of the stylohyal, and 

 by the confluence of the basi-hyals with the ceratohyals : a long glossohyal 

 is articulated to the upper part of the ligamentous symphysis, and a long 

 compressed urohyal to the under part of the same junction of the hyoid arch. 

 The glossohyal is wanting in the Murcenophis. 



The appendages of the hyoidean arch in fishes retain the form of simple, 

 elongated, slender, slightly curved rays, articulated to depressions in the outer 

 and posterior margins of the epi- and cerato-hyals : they are called " bran- 

 chiostegals," or gill-cover rays, because they support the membrane which 

 closes externally the branchial chamber. The number of these rays varies, 

 and their presence is not constant even in the bony fishes : there are but 

 three broad and flat rays in the carp ; whilst the clupeoid Elops has more 

 than thirty rays in each gill-cover : the most common number is seven, as 

 in the cod (fig. 30, 44). They are of enormous length in the angler, and 

 serve to support the membrane which is developed to form a great receptacle 

 on each side of the head of that singular fish. 



Branchial Arches. — In the class of fishes, certain bony arches, which ap- 

 pertain to the system of the visceral skeleton, succeed the hyoidean arch, 

 with the keystone of which they are more or less closely connected. Six of 

 these arches are primarily developed, and five usually retained ; the first four 

 of these support the gills, the fifth is beset with teeth and guards the opening 

 of the gullet : this latter is termed the ' pharyngeal arch,' the rest the ' bran- 

 chial arches.' 



The lower extremities of these arches adhere to the sides of a median chain 

 of ossicles, which is continued from the posterior angle of the basi-hyal, or 

 from above the uro-hyal, when this is ossified : the bones which form those 

 extremities are the ' hypobranchials'; and they support longer bent pieces. 



