56 CLASS PISCES. 



is called homocercal. In many Teleostei the tail fin of the larva 

 begins diphycercal then becomes heterocercal and finally assumes 

 the homocercal form. This correspondence between the develop- 

 mental history of the tail and the three forms of tail fin found 

 in living fishes is supposed to be highly significant from an 

 evolutionary point of view, for it is supposed that the diphy- 

 cercal tail is the most primitive, and that the homocercal is the 

 most specialised, the heterocercal tail intervening between 

 the two. This supposition is to a certain extent borne out by 

 palaeontology, which seems to show that Teleosteans are the 

 most modern group of fishes. Unfortunately for the theory, 

 however, the oldest fishes known to us had heterocercal tails and 

 not diphycercal, as the theory requires. 



In addition to these three types of tail-fin, intermediate conditions have 

 been named. For instance, the term heterodiphycercal has been applied 

 to slightly heterocercal tails in which the fin is much less developed on 

 the dorsal side than on the ventral (some Crossopterygians, Fig. 104), 

 while tails, in which the tail fin is externally symmetrical, but the 

 hind end of the vertebral column is bent and extends some way 

 into the dorsal lobe of the fin (Amia, etc.), are called hemiheterocercal 

 (Figs. 107, 109). The true homocercal tail is distinguished from the 

 hemiheterocercal by the fact that the vertebral column, which is bent 

 dorsalwards, does not extend into the fin, but terminates in front of it. 



As has already been mentioried, the dermotrichia of the ventro-caudal 

 fin of all Pisces are attached directly to the haemal arches (for apparent 

 exceptions to this see p. 55). In the homocercal tail of the Teleostean 

 these haemal arches are called the hypvral bones and are frequently fused 

 together to form a single broad plate of bone. In Ganoids with hetero- 

 cercal tails, when the upper lobe of the caudal fin (dorso-caudal) disappears 

 it is replaced by a series of ridge scales : the " fulcra " of palaeontologists : 

 in Elasmobranchs, when absent, it leaves no trace. 



The pectoral and pelvic fins also possess dermotrichia (fin-rays) 

 and somactids (radialia). A certain number of the somactids 

 are directly articulated to the limb girdles, and are then called 

 basalia. There are usually three of these, which are then called 

 pro- meso- and meta-pterygium, but their number varies con- 

 siderably. The important point to notice is the arrangement of 

 the peripheral somactids.* In Cladoselache, a Palaeozoic fish, 

 these are parallel to one another (Fig. 83), and the fin-skeleton 

 may be termed orthostichous.^ In most fishes, and notably in 



* Wiedersheim, Das Gliedmassenskelet der Wirbelthiere, Jena, 1892. 

 f The same feature is found in the pelvic fins of the Ganoid Psephurus 

 (Regan, Ann. and Mag. Nat. Hist (7), 13, 1904, p. 333.) 



