ROOM V. 



TAILS OF FISHES. 



421 



caudal fin is attached symmetrically ; and its figure is either 

 rounded, or divided into two equal lobes or branches ; these 

 tails are termed homocercal, i. e. even-tail, as is shown in Lign. 

 83, fig. 2. 



LIGN. 83. ILLUSTRATION OP THE STRUCTURE OP THE TAIL IN FISHES. 



1. Heterocercal tail ; a, b, the vertebral column extending into the 



upper lobe ; of a STURGEON. 



2. Homocercal tail ; of the A.yGVA.s.sET(Mallotusvillotus) of Green- 



land, a, vertebral column. 



In the second modification, the vertebral column, towards 

 its extremity, diverges from a straight line, and rises up, and 

 is prolonged into the upper lobe of the tail ; the caudal fin 

 appearing like a rudder, and the lower lobe being propor- 

 tionably very feeble and small, as in the Shark and Dog-fish ; 

 (Lign. 83, fig. 1 :) this form of tail is called heterocercal, i.e. 

 unequal- tail. But few of the existing fishes have this condition 

 of the caudal fin, while it is found in all the fossils that 

 occur in the ancient secondary strata ; namely, the Magnesiau 

 limestone, and antecedent deposits. The rounded, and equally- 

 bilobed or homocercal tails, are seen in many of the beautiful 

 fishes from the Chalk (Lign. 95); and the rudder-like, or 

 heterocercal tail, is shown in many of the Ichthyolites from 

 the Carboniferous strata (Lign. 85). 



GANOID ORDER. The fishes of this order are distinguished 

 by their brilliant angular scales, composed of osseous or 



