86 
freely grouping under it different species that belong to it, and which, up to 
the present, have been classed, some under Lucina, some under Fullastra, some 
under JJngulina and even under Tosidomya ; in fact, there may be united 
under it a number of Palaeozoic bivalves without external ligament or 
lunnle, orbicular or slightly ovate in form, little convex, with the valves quite 
closed, rather sharp, with small snh-central beaks little incurved, and the 
surface usually ornamented with more or less marked concentric lines of 
growth. The internal structure is very similar to that of Luciua. The casts 
being provided with a strong groove on the cardinal margin, demonstrate that 
the hinge must have attained a rather large size. 
Relations and Differences. — The species of this genus have a great 
likeness to Lucina, from Avhich they are distinguished chiefly by the entire 
absence of external ligament and lunule. 
Horizon. — All the species that I think should he referred to the genus 
Raracyclas, belong either to the Devonian or to the Carboniferous. These 
are Raracyclas {Lucina) proavia, Goldfuss, which I take as ty2:)e ; Dufrenoyi, 
d’Archiac and de Verneuil ; oliioensis. Meek, aniiqua and lineata, Goldfuss ; 
Raracyclas {Rullastra) elliffica, Phillips and R. {Venulites) concentrica, F. 
Ptoemer, that arc Devonian ; and Raracyclas coyana, A. d’Orbigny {Lucina 
aniiqua, McCoy, non Goldfuss) and hibernicensis, A. d’Orhigny {JJngulina 
antiqua, McCoy) that are Carboniferous. 
Paracyclas elliptic a, J. Rhillips. 
Rullaslra clliplica, J. Phillips, 18-40, Pal. Poss. Cornwall, p. 35, pi. 17, fig. 51. 
Cardinia ,, A. d’Orhigny, 1850, Prodr. Pal., I, p. 76. 
Rullastra „ J. Morris, 1851, Cat. Brit. Foss., p. 222. 
Shell inequilateral, suhelliptic, valves fairly regularly, though slightly, 
arched in the median portions, and depressed towards the cardinal and anterior 
Imrders, which are rather sharp. The beaks, very small, are contiguous and 
curved posteriorly. The entire surface is covered with concentric lines of 
growth, the depth of which, although slight, is rather variable in the space 
that separates them from one another, and thus produces a design hut little 
remilar. 
O 
Dimensions. — Length, fourteen millimetres; width, sixteen millimetres; 
thickness, six millimetres. 
