586 BRITISH PAL/KOZOIC FOSSILS. [ Pisces 
Genus. DIPLOPTERAX. 
= Diplopterus (Ag. not of Boie). 
Gen. Char.—Very large, elongate, fusiform fishes; two subtruncate dorsals, opposite two similar anal 
fins, they are about their own length apart ; numerous rays of the caudal above as well as below the very 
slightly elevated lobe of the tail; the caudal fin is obtusely pointed nearly in the middle, (and not forked nor 
obliquely truncated, as figured by Agassiz, in his restored figure) ; teeth nearly equal in size, simply conic, not 
plaited, and having a simple pulpy cavity; branchiostegous rays, replaced by two long subtrigonal plates 
under the throat, not quite meeting along the middle ; scales large, simply rhomboidal, very minutely and closely 
punctured, (merely fastened by their edges and external integument ‘); lateral line elevated. 
As the name Diplopterus of Agassiz had been previously used for another genus, it is necessary to give 
a new termination to the present group. 
DIPLOPTERAX AFFINIS (Ag. Sp.) 
Ref.— Ag. Old Red, t. 31a, fig. 27 and 27 a. 
The small fragments which are yet known of this species have a strong resemblance to the D. macro- 
cephalus, but with a much closer and more minute puncturing of the surface. 
Position and Locality.—Old Red sandstone—Gamrie. 
DipLopTERAX AcGassizit (Trail Sp.) 
Syn. and Ref.—Trail, Trans. Roy. Soc. Edinb. Vol. XV. =D. borealis, Ag. Old Red. t. 18. 
Desc.—Ahbout fifteen inches long and three wide, tapering insensibly from the head to the tail, which is 
nearly half the greatest width of the body; head semielliptical, one-fifth the entire length of the fish, depth 
about the same; scales more closely attached than in the D. macrocephalus, rhomboidal, one-third higher than 
long, the anterior and posterior margins straight ; upper margin convex, and lower coneaye, with the inferior 
angles slightly produced; those of the flanks about two and half lines high, and two lines long, acutely 
rhomboidal, with their length and breadth nearly equal; surface finely punctured; dorsal fins small, narrow, 
pointed ; first anal longer than deep, second shorter, and one-third deeper than the first; caudal fin rather 
large, slightly pointed in the middle, very slightly unsymmetrical, nearly as much fin above as below the 
spinal extremity, which does not reach to the end of the fin. 
Position and Locality —Old Red sandstone, abundant at Orkney. 
DIPLOPTERAX GRACILIS (M°Coy). Pl. 2. C. fig. 1. 
Ref —M°Coy, Ann. Nat. Hist. 2nd Series, Vol. II. 
Desc.—V ery slender, depth about one-eighth of the length, and nearly equal from head to tail; head nar- 
row, subtrigonal, obtusely pointed, about one-fourth longer than wide ; pectorals placed rather far back, small, 
of the preceding normally formed vertebra; in the “ heterocercal” type the vertebrae do not coalesce into a terminal 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 fin are developed from the under side only, and are intercalated with the spinous processes of a 
great number of vertebrae; the “diphycereal” type agrees with the “homocercal” 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 “heterocereal,” while it agrees with the latter and differs from the former in the gradual attenuation of the 
spinal prolongation, the terminal vertebra not being anchylosed into a vertically dilated mass, and the rays of the caudal 
being manitestly connected with the spinous 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 jishes 
have not exclusively heterocercal or embryonic types of tail, as was hitherto supposed. 
