Structure of the Sc fifes of Fishes. 2^ 



were, iuto small fragments. Such, at least, is the condition of the focus 

 in the greater part of the hard scales of the Acanthopterygians ; in the 

 Malacopterygians, on the contrary, and especially in membranous scales, 

 it often presents a circumscribed uniform surface, •without corpuscles, 

 and without interrupted circular lines. In such cases it is most frequently 

 surrounded with concentric cellular lines. 



The size of the focus varies ; sometimes it occupies only a very small 

 space, at others it is very extensive ; it determines the dimensions of 

 the different fields, because the lines of demarcation pass through a 

 centre. Its form is sometimes round, and sometimes oblong, elliptical, 

 square, &c. 



We term that a granular focus which is occupied by corpuscles, nume- 

 rous interrupted cellular lines, and cells more or less distinct, filled and 

 varying in shape ; that, on the contrary, we call a smooth focus, which 

 has neither interrupted cellular lines, filled cells, nor corpuscles, on its 

 surface. 



The greater part of granular foci are situate beyond the centre, so as 

 to render the basal field the largest, and the terminal one smallest ; the 

 majority of smooth foci, on the contrary, are placed in the centre, or very 

 little beyond it ; the fields are, therefore, more equal, and the form of the 

 scale is in general oblong, with concentric cellular lines round the focus. 



Teeth of Scales. The asperities found on the terminal edge of scales 

 have not, till now, been subjected to a detailed examination. We must 

 study these appendages in detail, in order to enable us to understand the 

 various changes which they undergo. 



The terminal edge of a scale of Corvina nigra is occupied by bodies 

 whose form is represented at fig. 15, which affords a Wiew of only a part of 

 the terminal field. At the first glance we can perceive that their bodies 

 are surrounded by an envelope. It was necessary, at the outset, to satisfy 

 ourselves whether this envelope, and the body in its interior, were really 

 separated from each other. This we ascertained by carefully tearing the 

 parts asunder by means of a fine needle. In so doing these bodies them- 

 selves are sometimes torn, at other times the envelopes only are injured 

 by being more or less deeply lacerated. We can thus remove almost all 

 these bodies, and the membrane in wliich tliey are placed remains by 

 itself. 



Examined by themselves, we soon perceive in these bodies an organi- 

 zation similar to that observed in teeth ; we first see a germ surrounded 

 by a sac ; this germ begins gradually to develope itself, acquires roots, 

 and we can distinguish different layers in it ; tiie extremity is unequal, 

 tnmcated, or pointed. We conceive, therefore, that ue may with pro- 

 priety call these bodies the tectii of scales, even with a reference to the 

 mode of their formation. 



These teeth arc least developed near the focus, and most highly in the 

 first series, that is to say, in tiie series nearest tiie terminal edge. We 

 liave au example in fig. 16 of the two upper teeth in their sacs ; tho roots 

 ate very difitiuct ; wc perceive the tliicknes? of the base of the tootli, and 



