PISH OF THE OLD RED SANDSTONE. 37 



mass, but diminish gradually in size to the last, forming a very 

 slender prolongation of the body, inclining upwards, and the rays 

 of the caudal tin being developed from the under side only, and 

 being intercalated with the spinous processes of a great number 

 of vertebrae ; the " diphycercal " type agrees with the " homo- 

 cereal '' in the nearly mesial position of the termination of the 

 body, and the nearly equal development of the caudal fin above 

 and below ; in those points it differs from the " heterocercal," 

 while it agrees with the latter and differs from the former in the 

 gradual attenuation of the spinal prolongation, the terminal ver- 

 tebrae not being anchylosed into a vertically dilated mass, and 

 the rays of the caudal being manifestly connected with the spi- 

 nous processes of a large number of vertebrae. Those who think 

 the theory of ^ progressive development ' worth refuting may be 

 glad to find that some of the oldest-known perfect remains of 

 fishes have not exclusively heterocercal or " embryonic '' types of 

 tails as was hitherto supposed. 



Diplopterus gracilis (M^Coy). 



Sp. Char. Very slender, depth about one-eighth of the length, 

 nearly equal from head to tail ; head narrow, subtrigonal, ob- 

 tusely pointed, about one-fourth longer than wide ; pectorals 

 placed rather far back, small, narrow, ovate, about three-fourths 

 the depth of the body, their base covered with scales as large 

 as those of the tail, but more square ; dorsal and anal fins 

 nearly equal, opposite each other, their own length apart ; they 

 are triangular and pointed, their height exceeding their length ; 

 tail diphycercal, of moderate size ; attenuated prolongation of 

 the body nearly medial, the caudal fin itself being rhomboidal, 

 pointed in the middle retrally ; scales rhomboidal, those of the 

 flanks about 2~ lines high and 1 J long, those of the tail smaller, 

 more obliquely rhombic, but still with nearly equal sides ; all 

 appear under the lens minutely and closely punctured, the 

 under side with a strong mesial articular ridge extending about 

 two-thirds the length. Total length about 14 inches, head 

 about one- seventh of the length. 



Its small head and extremely narrow elongate form easily di- 

 stinguish it from the other species. 



Not uncommon in the old red flags of Orkney. 

 [Col. University of Cambridge.) 



Osteolepis brevis (M'Coy). 



Sp. Char. Very short, fusiform, length about 5 inches, body 

 broadest at the anterior part, where the width is rather more 

 than one-fourth the length, tapering abruptly to the tail, the 

 pedicle of which is about one-third the depth of the body ; 



