FINS— TAIL—SCALES. 519 



Two types of fish fin are distinguishable — (a) that best ilhistrated 

 among living fishes by Ceralodits, in which a median jointed axis bears 

 on each side a series of radial rays — a form often called an archiptery- 

 gium ; and [b) the commoner type, in which the radials arise from a 

 number of basal pieces (an ichthyopterygium). Experts do not seem to 

 have yet come to a decision as to which of these types is the more 

 ancient, or as to how they are related to one another. 



Professor Huxley suggested that the fingered limb (cheiropterygium) 

 of higher Vertebrates might arise from a limb of the Ceratodiis type by 

 an atrophy of its proximal fore-and-aft radials, and the hypertrophy of 

 its distal radials. Thus the axis becomes the middle digit, while the 

 other four digits are the terminations of the two distal radials on each 

 side. But it seems just as easy or as difficult to trace the digitate limb 

 to an ichthyopterygium. 



Another interesting subject of inquiry is as to the origin of the girdles, 

 whether as ingrowths from the bases of the limbs, or from modifications 

 of branchial arches, or from both or neither. 



Tail. — In Dipnoi and a few Teleosteans, e.g.^ the eels, the vertebral 

 column runs straight to the tip of the tail, dividing it into two equal 

 parts. This perfectly symmetrical condition is called diphycercal or 

 protocercal, but it is not quite certain that its thorough symmetry is 

 primitive. 



In Elasmobranchs, Holocephali, cartilaginous and many extinct 

 Ganoids, the vertebral column is bent dorsally at the end of the tail, 

 and the ventral part of the caudal fin is smaller than, and at some little 

 distance from, the upper part. This asymmetrical condition is called 

 heterocercal. 



In most Teleostei, and in extant bony Ganoids, the end of the verte- 

 bral column is also bent upwards, but the apex atrophies and, by the 

 disproportionate development of rays on the ventral side, an apparent 

 symmetry is produced. The vertebral column usually ends in a urostyle, 

 — the undivided ossified sheath of the notochord. Most of the fin really 

 lies to the ventral side of this. The condition is termed homocercal. 



Sialus. — In Elasmobranchs the scales (placoid) have the form of skin 

 teeth (dermal denticles), tipped with enamel, cored with dentine, and 

 based with bone sunk in the dermis. They arise from skin papillse, the 

 (ectodermic) epidermis forming the enamel, the (raesodermic) dermis 

 forming the rest. It has been recently maintained, however, that the 

 ectoderm forms most, if not all, of the scale (see p. 426). In other 

 fishes the scales are almost wholly dermic, in marked contrast to those 

 of Reptiles. 



In most Teleosteans the scales are soft, and the epidermic covering is 

 very thin. They are called cycloid or ctenoid, as their free margins 

 projecting from sacs in the dermis are entire or notched. But bony 

 scales also occur in many Teleosteans. 



The sturgeon has five rows of bony dermic plates (scutes) ; the scales 

 of the Bony Pike [Lepidosteiis], Poiypterus, and many extinct Ganoids 

 are covered with enamel. 



The great interest of these exoskeletal structures is that those of Elas- 

 mobranchs are homologous with teeth, and that many bony scales often 

 fuse into plates, suggesting the manner in which the membrane bones of 



