596 BRITISH PALAXOZOIC FOSSILS. [ Pisces. 
has them (in the same parts of the body) smaller, more rounded, and the exposed portions much less high in 
proportion to their length. 
Position and Locality.—Not uncommon in the Old Red sandstone flags of Orkney. 
Explanation of Figures.—PI. 2. D. fig. 6, natural size; the dotted restored outlines of the fins are from 
other specimens in the collection ; fig. 6 a, scales of ditto magnified one and half diameters. 
Genus. GYROPTYCHIUS (A/° Coy). 
Ref—M Coy, Ann. Nat. Hist. 2nd Series, Vol. IT. 
Gen. Char.—Slender elongate ganoid fishes, with large, semioval, depressed heads, from which the body 
gradually tapers to the tail, which is diphycercal, the caudal fin being rhomboidal, pointed in the middle of the 
retral margin, and the prolongation of the body extending a little above the medial line, nearly to the end; two 
small elliptical dorsal fins exactly opposite two similar anal fins; pectoral fins broad, rounded, placed rather 
far back. Scales of the flanks subrhomboidal, nearly oval on the back, imbricated, the exposed portion of each 
with minute rough ridges, which converge towards the retral end, and seem to gyrate round a nearly central 
point ; the anterior concealed portion nearly smooth (or under a strong lens minutely radiated as in Holoptychius) ; 
under surface of each scale nearly smooth, with a strong mesial keel, which extends from the anteal edge, only 
as far as the central point, where it abruptly terminates to allow of the imbrication of the remainder of the scale 
on the next behind; bones of the head covered with granules, which are sometimes confluent into short ridges ; 
teeth small, conical, nearly equal. 
In form, number, and position of the fins, and structure of the tail, these fishes resemble Diplopterax, while 
the imbrication of the scales, as well as their sculptured, instead of simply porous surface, places them close to 
Holoptychius, and in a different family of Agassiz system from the former; while from the latter they differ in 
form and number and position of the fins, structure of the tail, and in the ridge on the under side of the scales, 
which reminds us of what we see in Osteolepis and many other fishes with juxtaposed scales; but instead of 
extending entirely across the scale it only reaches half-way, the half-ridge of one scale joining that of the next 
behind and before when in their natural imbricated position. The Gyroptychii are thus intermediate between 
Holoptychius and Diplopterax, and serve to connect the great groups of Calacanth and Sauroid fishes to 
which those genera respectively belong, haying at the same time a style of sculpturing of the scales peculiar 
to themselves and easily recognizable. 
There are two imperfectly known and imperfectly characterized genera of M. Agassiz, Glyptopomus and 
Platygnathus, which require a few words in connexion with the present fishes. The first of those genera is 
founded on a short, thick, fusiform, sauroid fish with simply juxtaposed, rhomboidal, granulated scales; the 
fins being nearly unknown. The slender form of the present fishes, and the shape, sculpturing, and im- 
brication of the scales are sufficient distinctions therefrom, the fins being unknown. Platygnathus is a genus 
founded by Agassiz on the jaw of one fish and the tail of another; the jaw agrees nearly with Bothriolepis, 
but has fewer laniary teeth, and needs no comparison with Gyroptychius; the tail portion is more analogous, 
but the great scales seem to have simply the structure of Holoptychius, and the fins are developed beyond all 
comparison; further, this genus is founded on such imperfect and perhaps discordant materials, and the jaw 
seeming to have been most in view in naming and defining the group, it is obvious that even an identity 
between one of those elements and the present perfectly known type would not invalidate Gyroptychius as a 
genus. Judging from the figure and description of M. Agassiz, however, there seems, as above noted, to be 
no great affinity between them. 
Gyroprycuius anaustus (M/°Coy). Pl. 2. C. fig. 2. 
Ref—M ‘Coy, Ann. Nat. Hist. 2nd Series, Vol. II. 
Desc.—Head semioval, obtusely pointed, about one sixth the entire length; depth of the body greatest 
immediately behind the head, where it is one-seventh of the length, tapering very gradually from thence to 
