478 BRITISH PAL/ZOZOIC FOSSILS. [LAMELLIBRANCHIATA. 
Genus. AMUSIUM (Megerle). 
= Pleuronectia (Sw.) < Pecten (Brug.) 
Gen. Char.—Ovate, depressed, nearly equivalve, perfectly equilateral ; ears small, equal, their upper angle 
elevated above the level of the beak, so that the hinge-line forms a re-entering angle, along which is a narrow 
facet for the external ligament, and under the beak a small triangular pit for the internal cartilage; two long, 
diverging, internal ridges from the beak in each valve, originating within the tooth-like processes; margin 
gaping at the sides; surface without radiating ridges. 
The recent Pecten pleuronectes forms the type of Swainson’s genus Plewronectia, but the smooth Triassic 
fossil species previously separated by Sch/otheim, under the name Plewronectites, were true Pectens of the type of 
the recent P. obsoletus (with deeply notched anterior ear and straight hinge). I use the generic name Amusium 
(= to Plewronectia), although many years ago I separated in MSS the fossil under the name Jsopecten. 
It is remarkable that so peculiar a generic type should not have been generally recognised ; the species differ 
from Pecten in being equilateral, and their hinge-line, instead of being straight, forming at the beak a re-entering 
angle; an arrangement which impeding the opening of the valves is compensated for by the gaping of the margins, 
which only touch at the hinge and opposite point of the circumference, while in Pecten they are closed all round, 
except where one ear is notched for the byssus; there is no notch in the present genus. From the similarity of 
shape and smooth or concentrically lined surface of all the species, they are very difficult to discriminate. 
AmustuM? DEORNATUM (Phill. Sp.) 
Ref. and Syn. =Pecten deornatus Phill. Geol. York. t. 6. f. 26. 
Dese.—Ovate, apical angle about 105°, slightly convex, most so near the beaks; ears small, equal, nearly 
square; surface with irregular, small, concentric, obtusely-rounded, smooth wrinkles, obsolete at the sides (five 
in the space of one line in the middle of the shell). Width from beak to opposite margin four lines, length 
slightly less. 
The only specimen I have at hand of this rare species is rather imperfect about the beak and ears, yet I 
think I see traces of the cartilage pit and elevation of the ears, sufficient to warrant my placing it provisionally 
in this genus. It differs from the Amusiwm Sowerbyi (M°Coy) in its smaller size and irregular, concentric, obtuse 
wrinkles, and from the young of the Pecten Sibericus (de Ver.), by the same points and the form of the ears. 
Position and Locality—Rare in the carboniferous limestone of Lowick, Northumberland. 
AmusituM Sowerby! (M°Coy). 
Ref. and Syn. = Pecten Sowerbyi M*Coy (not Nyst) Syn. Carb. Foss. Irel. t. 14. f. 1.= Peeten Bathus 
D’Orb. Prod. p. 139. 
Desc.—Elliptical ; apical angle 110°, slightly convex in the middle of the rostral third, most so at one- 
third the length from the beak, gradually arching to the front margin and to the apex, but the sides abruptly 
flattened, or slightly concave in the rostral half; ears small, equal, acute, conate, considerably elevated above 
the beak, defined from the body of the shell by a sharp sulcus on each side; surface finely striated concen- 
trically, eight strize in one line in the middle of the shell. Width from beak to opposite margin one inch one 
line, proportional length 44, depth of one valve 7%, length of ears from beak to apex ib 
When the very thin superficial layer is removed a number of small interrupted, zigzag, and divaricating 
seratch-like markings appear; the divaricating internal ridges are very thick, diverge at an angle of 75°, and 
are two-thirds the width of the shell in length; the hinge-teeth are only one-third the length of those ridges 
and diverge at 105°, coinciding with the line of junction of the ears and body of the shell. The muscular 
impression is large, and a little on the posterior side of the middle and a little nearer the beak than the 
