454 



COMPARATIVE ANATOMY. 



The quadrate (Q) is segmented off from tlie palato- quadrate carti- 

 lage ; it carries tlie glenoid cavity for the mandible. The angulated 

 ectopteiygoid (Ept) is attached to the front of the quadrate ; and 

 between it and the hyomandibular there is the flattened and 

 ordinarily quadrangular metapterygoid (Mt). The entopterygoid is 

 nearer the middle line than the ectopterygoid ; finally the palatine 

 forms the most anterior end of the palato- quadrate cartilage ; it is 

 generally movably connected with the skull. 



In front of the palatine there are two other bones, which are 

 not preformed in cartilage; the hinder one, which is generally 

 attached to the palatine, is called the maxilla (Fig. 245, Mx), and the 

 anterior one the premaxilla (Px). They appear as new pieces, which 

 henceforward play an important part. But it is exceedingly probable 

 that they have their origin in the two upper labial cartilages of the 



Fig. 245. Side view of the cephalic skeleton of Salmo salar (cf. Fig. 242, A). 

 Fr Frontal. N Nasal, n Nasal pit. Pet Parietal. Sq. Squamosal, i i i i Infra- 

 orbital bony riug. Hm Hyomandibular. Sy Symplectic (shown as if it were visible 

 from the exterior). Mt Metapterygoid. Ept Ectopterygoid. Q Quadrate. Mm Max- 

 illa. Pre Premaxilla. Art Articular. Aug Angular. D Dentary. Op Operculum. 

 PrOp Preoperculum. Sop Suboperculum. Jop Interoperculum. Ug Ligament. 



Selachii. Sometimes they are freely movable, and oven protrusible ; 

 in other cases they are more firmly connected with the skull. This 

 is especially the case with the premaxilla, which is frequently united 

 firmly to the most anterior portion of the ethmoidal region. They 

 both bound the orifice of the gape, but when the premaxillas are 

 increased in length, the maxillae may be shut out from any share in 

 it; while conversely, when the premaxillas are shortened, the maxilla) 

 acquire a larger share in this relation to the mouth. 



In the lower jaw the cartilagiuous rudiment, or Meckel's 

 cartilage, is very completely retained. It gives rise to the dentary 

 (D), which, as it were, ensheaths the cartilage. The articular (Art) 

 is formed from the condylar portion of the cartilage, and below it 

 there is the angular (Ang). On the inner face of the bony lower 

 jaw a special bone is sometimes developed as a covering-piece for 

 the cartilage ; this is the opercular. 



