584 BRITISH PALAXOZOIC FOSSILS. [ Pisces. 
Genus. DIPLACANTHUS (Ag.) 
Gen. Char.—Body of moderate, or rather small, size, ovate: two dorsal fins ; the first or bony ray of one 
a little behind the line of the pectorals, and the bony ray of the other exactly opposite that of the anal fin; 
anal fin about the size and shape of the posterior dorsal ; ventrals smaller than the anal, placed slightly behind 
the middle of the body; pectorals moderately large, pointed; all the fins with a strong bony ray in front ; 
besides those connected with the fins there are usually a strong pair of bony spines projecting from the 
thoracic cincture under the throat, and a much simaller pair from the ventral line between the ventral and 
pectoral fins. 
The fins are, I think, truly covered with scales, and had each originally but the one ray, as the impression of 
the delicate sail-like membrane is often preserved perfectly. I have never seen the least trace of a bony vertebral 
column. 
DIPLACANTHUS CRASSISPINUS (Ag.) 
Ref —Ag. Old Red, t. 13. fig. 1 and 2. and t. 14. f. 6 and 7. 
Dese.—Length about three and half inches; depth about one inch; scales subrhomboidal, gibbous, nearly 
twice as high as long ; posterior margin rounded, about nine or ten in the space of two lines, surface obscurely 
granulated; defensive spines very strong, and coarsely suleated longitudinally, the dorsal with seven or eight 
furrows. 
Position and Locality—Common in the black flags of the Old Red sandstone of Caithness. 
DipLacaNtHus Gippus (M° Coy). Pl. 2. B. fig. 4. 
Ref.—M ‘Coy, Ann. Nat. Hist. 2nd Series, Vol. II. 
Dese.—Very thick, fusiform ; depth of the body behind the pectorals equal to the distance between the two 
dorsal spines ; the dorsal spines are equal, about one inch long and one line wide; the first dorsal less than the 
depth of the body at its base; anal spine much shorter than the second dorsal; scales rhomboidal, length and 
width nearly equal, each with a large, vertically diagonal, oval gibbosity, occupying nearly its entire area, about 
five scales in one line; surface seems very minutely granular. Length about four and half inches; greatest 
depth one and half inch. 
This species has more equal-sided scales than the D. striatus (Ag.), and is of nearly double the proportional 
depth ; the spines are longer and more slender than in the D. crassispinus (Ag.), beside the difference in the 
scales. 
Position and Locality.—V ery rare in the Old Red flags of Orkney. 
Explanation of Figures —P\. 2. B. fig. 4, natural size of specimen, wanting the head. (The small patch 
of scales does not clearly shew their mesial prominence) ; fig. 4 a, scales of do., from near the middle, magnified 
24 diameters. 
DIPLACANTHUS LONGISPINUS (Ag.) 
Ref—aAg. Old Red, t. 18. fig. 5. and t. 14. fig. 8 and 9. 
Dese.—Length about eight or nine inches; greatest depth of body two inches nine lines ; bony spines of 
the anal and dorsal fins very strong, about equalling the depth of the body at their bases (measured along 
the profile of the back) ; upper lobe of the tail rather short and wide ; scales large, convex, rhomboidal; anterior 
margin rounded, posterior margin obtusely pointed, surface with very fine numerous stri converging towards 
the posterior apex; about seven scales in a quarter of an inch at the middle of the body. 
Position and Locality.—Not uncommon in the Old Red sandstone of Lethen Bar. 
