DERMAL BONES OF FISHES. 145 



fins, and both are situated near the end of the tail, which runs into 

 the upper lobe of an unsymmetrical caudal fin. Now in the embryos 

 of existing Osseous Fishes these vertical fins are developed from a 

 single continuous fold of integument, which is extended round the 

 tail from the dorsal to the ventral surface ; a condition which we shall 

 see in the tadpoles of Batrachia, and which is persistent in the Eel 

 and Lepidosii'en. The growth of this fold is progi'essive at certain 

 parts and checked at others ; and where development is active the 

 supporting dermal rays make their appearance, and the transform- 

 ation into dorsal, anal, and caudal fins is thus effected. At first the 

 caudal fin is unequally lobed and the terminal vertebras extend into 

 the upper and longer lobe ; the dorsals and anals are also, at first, 

 closely approximated to each other and to the caudal fin. M. Agassiz 

 has shown that all these embryonic characters were retained in 

 many of the extinct fishes of the Old Red Sandstone ; and the de- 

 velopment of the caudal fin did not extend in any fish beyond the 

 heterocercal stage until the preparation of the earth's surface* had 

 advanced to that stage which is called Jurassic or oolitic in geology, 

 (xxii. fasc. Sur le Systcme Devonien^ 



Teleology of the Skeleton of Fishes. — Thus far the osteology of 

 Fishes has been considered chiefly from a homological point of 

 view, and I have aimed at relieving the dryness of descriptive de- 

 tail, and at connecting the multifarious particulars of this difiicult 

 part of Comparative Anatomy in natural order, so as to be easily 

 retained in the memory, by referring to the relations which the 

 skeletons of Fishes bear to the general plan of Vertebrate organisa- 

 tion, and by indicating their analogies to transitory states of the 

 embryo skeleton in higher animals, and to those answerable conditions 

 of the mature skeleton which, in longer lapse of time, have successively 

 prevailed and passed away in the generations of species that have left 

 their remains in the superimposed strata of the earth's crust. 



To determine the parts of the Vertebrate skeleton which are mos 

 constant ; to trace their general, serial, and special homologies, 

 under all the various modifications by which they ai'e adapted to the 

 several modes and spheres and grades of existence of the different 

 species, should be the great aim of osteological science ; as being that 

 which will reduce its facts to the most natural order, and their ex- 

 position to the simplest expressions. It is impossible, in pursuing the 

 reiiuisite comparisons upwards through the higher organised classes, 

 not to recognise the close and interesting analogies between the 

 mature states and forms of ichtliyic oi'gans, and the embryonic condi- 



* " The sea is His, and lie made it, and His hands prepared tlie dry land." — 

 Fs. xcv. 



VOL. II. L 



