386 Mr. J. T. Cunningham and Dr. C. A. MacMunn. [Apr. 27, 



Gurnard, Cottus, Pipe-fishes, &c, that the general distribution of the 

 elements is constant in all, the differences being in minute details. 

 Thus, there is always an argenteum consisting of reflecting tissue 

 composed of needles or granules in a layer of greater or less thick- 

 ness. And in the superficial region of the skin there is a layer of 

 chromatophores associated with a thinner deposit of reflecting tissue 

 corresponding to the iridocytes of the Pleuronectid. Thus, in the 

 Herring the superficial reflecting tissue is in the form of a layer o 

 slender rods or prisms lying side by side and adhering to the inner 

 surface of each scale, forming a coating to the latter when it is 

 removed, and endowing it with its beautiful iridescence. The scales 

 themselves are never silvery or iridescent. In the Herring the 

 argenteum consists of similar rods in close apposition, forming 

 lamina?. On the dorsal surface of the body, where the argenteum 

 becomes thinner, chromatophores are found on its surface and pene- 

 trating it with their processes; on the ventral surface these are 

 absent, and the argenteum is very thick. 



The ultimate histological relations of the coloration elements we 

 have not completely elucidated, but have merely described the rela- 

 tions as seen in sections, pointing out the difficulty of regarding the 

 iridocytes of the Pleuronectids as cells, considering their homology 

 with the reflecting tissue in other forms, such as the Herring, and 

 the fact of the argenteum, a continuous deposit whose cellular nature 

 is very improbable, being developed from iridocytes. 



In chemical and physical properties the substances contained in 

 the coloration elements are as distinct as the elements are in appear- 

 ance and form. The black chromatophores owe their colour to a 

 melanin which is granular in its natural condition, is a nitrogenous 

 body, and is very refractory towards reagents. The pigment of the 

 coloured chromatophores is always a lipochrome, and the absorption 

 bands of the various lipochromes obtained from the Eishes examined 

 do not differ to any great degree. The reflecting tissue was found 

 always to consist of guanin in the pure state, not, as has often been 

 stated, to a combination of guanin and calcium. The differences in 

 the appearance of the reflecting tissue in the natural state, whether 

 it is silvery, chalk white, or iridescent, depend on the form of its 

 minute elements. It is chalk white when granular, silvery when 

 composed of very fine needles in a thick layer, iridescent when com- 

 posed of thicker prisms in a thin layer. The opacity and reflecting 

 power is a property of the guanin itself in any form. Besides these 

 substances, large crystals of phosphate of calcium were found in 

 many skins, both of Pleuronectidee and other Fishes, and from these 

 or from the scales has probably been derived the calcium supposed to 

 be associated with the guanin. 



These investigations of the elements and substances of coloration 



