Zoology, 637 
arches do not articulate with each other, but that in all remaining 
Elasmobranches there is a suspensorial arrangement. In Centrophorus, 
Mustelus, and Acanthia there is no direct contact of the two arches ; 
in Oxyrhinia and Sphyrna the hyoid and mandible have developed 
articular facets for contact, whilst the hyomandibular does not ; but in 
Galeus, Scymnus, Cestracion, and Trygon the hyomandibular and man- 
dible are in contact. In Trygon the former is also in contact with the 
quadrate portion of the first arch. 
The Dipnoan and Batrachian series show the gradual and finally ab- 
solute estrangement of the hyomandibular-hyoid arch from the 
palato-quadrato-mandibular arch, leading to the loss of ligamentary 
connection, and to the final attachment of the hyoid to the cranium. 
The hyoid becomes completely separated from the hyomandibular, 
which would have aborted completely had it not assumed new — namely 
auditory — functions, by becoming connected with a tympanum, i.e., 
a cavity formed out of the first visceral cleft. The hyomandibular, in- 
vested with this new function, breaks up into two or more pieces, as an 
ossicular chain. The old piscine ligamentous, or even cartilaginous, 
connection between hyomandibular and mandible is lost in the Salientia, 
and in the Urodela a piece of cartilage, comparable either with a sym- 
plectic or an opercular element, is also gradually lost. The tympanal 
end of the auditory chain or rod becomes connected with the crani- 
um by a suprastapedial element, probably of periotic origin, while the 
quadrate becomes reduced to a small cartilage wedged between the 
elongated pterygoid and squamosal. This elongation of the pterygo- 
quadrate bar, transposing the masticatory joint outwards, away from 
the cranium, has caused, or at least facilitated, the separation of the 
hyoid from the hyomandibular. In the Chelonia the broad quadrate is 
fused with the skull. In the Trionychidae and land tortoises (as 
shown by Peters) the quadrate forms a closed canal through which 
passes the columellar rod, but in other tortoises and turtles it forms an 
imperfect canal, open behind and below. In all chelonians the inter- 
fenestral apparatus consists of two pieces, and the hyoid is frequently 
either absent or is a mere bit of bone or cartilage attached to the basi- 
lingual plate. The pair of long bony bars which act as hyoid is 
really the third visceral or first branchial arch. In the Crocodilia the 
auditory apparatus is very complex. The air cavities of the os articu- 
lare are connected with the middle ear or tymiianic cavity by the 
fibrous and partly cartilaginous siphonium. The air-cavities of the 
quadrate are also in direct communication with the tympanic cavity. 
The outer end of the columella proper possesses a concave facet, by 
