924 



CLASS PISCES. 



between the cranium and the palatopterygoid bar ; but in Notidanus 

 (fig. 86 1 ) the hyomandibular element takes no share in the support 

 of the mandible, and the palatopterygoid articulates directly with 



the cranium ; this probably being 

 the primitive type. The gills (fig. 

 848) are attached to the skin by 

 their margins ; while they usually 

 communicate with the exterior by 

 five apertures, or clefts, which may 

 be very rarely increased to six or 

 seven. The mouth is very gener- 

 ally situated on the inferior aspect 

 of the body (fig. 849), and is fur- 

 nished with numerous teeth carried 

 on the palatopterygoid bar and 

 Meckel's cartilage (fig. 861). These 

 teeth may be either sharp and separ- 

 ate, or articulated together so as to 

 form a more or less pavement-like 

 structure; and in the former case 

 there is a continuous succession of 

 new teeth developed from behind 

 as the old ones are worn out. Both 

 median and paired fins are present ; 

 the position of the pelvic pair being 

 always abdominal. In all existing 

 forms the skeleton of the limbs 

 forms an ichthyopterygium (fig. 

 846) ; but in the Carboniferous and Permian Ichthyotomi there 

 is either a uniserial or biserial archipterygium, like that of the 

 Dipnoi. The posterior termination of the vertebral column is gener- 

 ally heterocercal, with the upper lobe of the caudal fin greatly elon- 



Fig. 848. — Diagram of one side of the gills 

 and gill-pouches in a Selachian ; the arrows 

 indicating the direction of the currents. 

 ba 1-5, Branchial arches transversely di- 

 vided ; be, Branchial pouches ; gs, External 

 gill-slits ; s, Septa between pouches ; br, 

 Branchiae. 



Spiny Dog-fish (Acanthias). Reduced. 



gated (fig. 849). The spines frequently borne by the dorsal fins 

 and in the nuchal region, constituting the so-called ichthyodorulites, 

 have their bases simply embedded in the flesh, and are consequently 

 immovable. There is no swim-bladder. The term spiracles is ap- 



