686 



GEOLOGY. 



Another remarkable circumstance pointed out by Agassiz refers to the diverse form 

 of the tails of fishes, as in the engraving, which represents 1. the homocercal, or even-tail, 

 as in the trout ; 2. the single and rounded, as in the wrasse ; and, 3. the heterocercal, or 

 unequal tail, as in the shark. Now it is a singular fact, that nearly all the fossil fishes 



Shark. 



Trout. 



VV rasse. 



that occur in the ancient strata, or below the magnesian limestone, are distinguished by 

 their heterocercal or unsymmetrical tails ; while this form is rarely found in the oolitic 

 and superior systems, and among living species. The heterocercal structure in the case 

 of the living shark is an arrangement to meet the position of the mouth, which is placed 

 downwards beneath the head, so that the body of the animal requires to be turned, in 

 order to bring the mouth in contact with its prey a movement to which the peculiar 

 form of the tail is adapted. As the heterocercal tail is formed by a prolongation of the 

 vertebral column, it seems an advance of organisation in fishes towards the class of rep- 

 tiles, in which the same structure is fully developed. 



When death closed the labours of Cuvier, and arrested the progress of his great work, 

 Ossemens Fossiles, he had only just commenced the difficult task of examining and 

 arranging the fossil remains of fishes, naming and describing but ninety -two distinct 

 species. Committing the materials he had collected to Agassiz, the number of species 

 was raised to 1600 in little more than fourteen years, and it is now rapidly verging 

 towards 2000. Some, the most strange and singular conformations, are encountered in 

 the old red sandstone, " creatures whose very type is lost fantastic and uncouth, and 

 which puzzle the naturalist to assign them even their class : boat-like animals, 

 furnished with oars and a rudder ; fish plated over, like the tortoise, above and below, 

 with a strong armour of bone, and furnished with but one solitary rudder-like fin ; 

 other fish, less equivocal in their form, but with the membranes of their fins thickly 

 covered with scales, creatures bristling over with thorns; others glistening in an 

 enamelled coat, as if beautifully japanned: the tail, in every instance among the less 

 equivocal shapes, formed not equally, as in existing fish, on each side of the central 

 vertebral bone, but chiefly on the lower side, the bone sending out its diminished 

 vertebrae to the extreme termination of the fin. All the forms testify of a remote 

 antiquity of a period whose fashions have passed away. The figures on a Chinese 

 vase or an Egyptian obelisk are scarcely more unlike what now exists in nature than the 

 fossils of the old red sandstone." Among these remarkable objects, the following 

 occur : 



Cephalaspis. This genus, of which four species have been described, is remarkable 

 for the great size of the head in proportion to the body, equal to fully one-third of the 

 entire length of the creature, and for its shield-like appearance, after which it has been 

 named, Cephalaspis buckler-head. The outline is that of a crescent, with the lateral 

 horns inclining slightly to each other. The eyes are placed in the middle of the shield, 

 close together, as is the case with many of the flat fish. Scales variously circular and 

 angular, bony, with an exterior enamelled surface, each having a convex centre, from 



