FISH OF THE OLD RED SANDSTONE. 41 



hind the head, where it is one-seventh of the length, tapering 

 very gradually from thence to the tail, the pedicle of which is 

 about one-third the greatest depth of the body; caudal fin 

 large, rhomboidal, obtusely pointed behind, lower side largest, 

 the supramedial spinal prolongation extending nearly to the 

 end ; posterior anal fin semi-elliptical, equalling the depth of 

 the body at its base in height, which is about double the 

 length ; both the posterior anal and dorsal fins nearly touch 

 the caudal, and are fully their own height behind the anterior 

 and dorsal fins, which are about one-third less in size ; the 

 pectoral fins are broad ovate, scarcely two-thirds the depth of 

 the body at their base in height, and placed nearly their own 

 height behind the head ; scales about 2 lines high (or wide) 

 and about one-third longer, elliptical and with a small, nearly 

 central boss, round which seem to gyrate very numerous mi- 

 nute rough ridges, less than their own diameter apart, which 

 are arranged in converging curved lines parallel with the mar- 

 gin of the elliptical free end of the scale ; the small portion of 

 the ridges anterior to the central boss are frequently broken 

 into little tubercles ; the more anterior concealed portion is 

 smooth or very minutely radiated, the (? articular) ridge on 

 the under side strongly marked ; bones of the head closely 

 sculptured with small granules and short vermicular ridges. 

 Length nearly 1 foot ; length of exposed portion of scales 

 slightly more than 2 lines. 

 This beautiful fossil much resembles the Diplopterus gracilis 



(M^Coy) in form, but is at once distinguished by the structure 



and sculpturing of the scales. 



Rare in the old red sandstone of Orkney. 



{Col. University of Cambridge — two fine examples.) 



Gyroptychius Diplopteroides (M'Coy). 

 Sp. Char. Head semi-elliptical, depressed, sides flattened, slightly 

 longer than wide, pointed in front, about one-fifth of the entire 

 length ; body tapering rapidly from the head to the tail, the 

 pedicle of which is less than half the width of the body ; cau- 

 dal fin rather large, rhomboidal, submedial spinal prolonga- 

 tion slender ; posterior dorsal elliptical, twice as high as long, 

 close to the base of the caudal, and reaching about half the 

 length of its lateral angle ; anterior dorsal little more than 

 half the size of the posterior ; pectorals short, broadly rounded, 

 placed rather more than their own length behind the head ; 

 scales of the back oval, imbricated ; concealed portion, anterior 

 to the subcentral point, smooth or very minutely radiato-punc- 

 tate, all the posterior or exposed portion rough with small, 

 irregular, minutely flexuous ridges, those of each side running 



