Class 2. Pisces. 357 



and ctenoid scales are distinguislied ; in tlie latter the posterior 

 edge is finely denticulate. Scales, which are especially frequent in 

 the Teleosteij are simply one form of dermal ossification, and are 

 not sharply demarcated from others, bony plates, scutes, spines, etc., 

 which are present in many Fish. The p 1 a c o i d scales (dermal 

 denticles) covering the whole surface of many Selachians, and 

 present also in various other Pisces, are entirely different. They are 

 identical in structure and development with the buccal teeth, consisting 

 of dentine and enamel formed in the ordinary way ; they do not lie 

 in the dermis, as do most dermal ossifications,* but their upper 

 portion projects from the skin ; they fall out and are replaced, whilst 

 dermal ossifications usually grow with the growth of the animal, and 

 are neither deciduous nor successional. The form of these dermal 

 teeth varies, sometimes they have many points ; usually they are 

 small, but may reach a considerable size. In those Teleostei and 

 G-anoidei which possess dermal denticles, their lower end is usually 

 connected with the dermal ossifications. t 



The unpaired fins are folds of skin usually supported by 

 hard parts. At a certain stage in development, frequently even in 

 the newly-hatched animal, more often during embryonic life, the 

 unpaired fins are represented by a continuous fold, which runs 

 medianly along the dorsal surface of the body and tail, round its 

 tip and ventral side, along part of the body. In certain cases, this 

 ridge remains undivided throughout life, but the ventral portion ia 

 front of the anus always disappears ; usually, however, it breaks 

 into three or more sections, of which those on the dorsal surface 

 are termed dorsal fins; that round the tail, the caudal fin; 

 and those ventral to the tail, anal fins. When fully developed, 

 hard portions, the so-called fin rays, are generally present. In 

 the Selachians, " horny rays "% occur ; horny, elastic, structureless 

 fibres, stretching from the base of the fin to the edge ; they lie in 

 several layers in each fin, which is stiff and incapable of folding. § 

 Instead of these, there are in the unpaired fins of Ganoids, Dipnoans, 

 and Teleosteans, a series of rod-like dermal ossifications, the bony 

 rays, which lie as supports within the fin. Of such rays, two chief 

 forms may be distinguished, soft rays and spinose rays, 

 between which there are, however, intermediate forms. A soft ray 



* Not infrequently, howeTer, the true dermal ossifications have a large surface, 

 or a projecting point, bare. 



t The fine tooth-like points along the hind border of the ctenoid scales, which are 

 Taerely special portions of these, are not to be confounded with the dermal denticles. 



X The name is not a very happy one, since it is here applied to parts which develop 

 in connective tissue, and are entirely distinct from the trvie horny structures of 

 Vertebrata. 



§ A series of cartilaginous rays occurs in the Cyclostome fin. 



