132 
PALAEONTOLOGY OF NEW YORK. 
We must therefore consider the enclosure of the groove an essentially adult 
character of Orbiculoidea, and in cases like that of the genus Trematis, where 
the substantial difference from Orbiculoidea lies in the persistent open pedicle- 
fissure, we may regard the generic distinction as founded not on a genetic, but 
rather on a developmental difference, Trematis retaining at maturity a pedicle- 
passage having a character, which in Orbiculoidea, is embryonic ; and as far 
as these characters alone are concerned, the same is essentially true of Schizo- 
crania. There are species passing under the name of Discina, which show such 
features as these at maturity; as for example, a form in the Lower Helderberg 
commonly referred to the Discina discus, Hall, of which the best specimens 
obtainable indicate that the aperture is similar to, but considerably more con¬ 
tracted than in Schizocrania, approaching Trematis in this respect; but the 
character of the upper valve, its marginal beak and finely radiated surface, show 
further agreement with Schizocrania, and it seems best to remove the species, 
provisionally at least, to this genus* 
Again, the Discina pleurites of Meek, from the Waverly sandstones of Ohio, 
is a species whose form and contour is precisely that of Schizocrania ; the 
upper valve is. evenly convex, and has a submarginal beak, but its surface 
markings consist wholly of concentric striae. As in Schizocrania, also, attach¬ 
ment is largely effected by the margin of the upper valve, while the pedicle- 
passage is an open fissure extending from the termination of the floor of the 
groove to the margin, where it is somewhat'constricted, as in Trematis; at the 
same time the floor of the pedicle-groove partakes of the tripartite structure 
seen in all these genera. The entire pedicle-area is conspicuously elevated, as 
in Orbiculoidea Newberryi, and it may be quite proper to regard these two species 
as forms in which the development of this feature has been arrested at an 
earlier stage in one ( D. pleurites ) than in the other. With our present compre¬ 
hension of the genera Schizocrania and Trematis, it is impossible to admit 
this species to either, for no specimen has shown a trace of muscular imprints, 
which are usually very strong in the upper valves of both these groups. 
* See in Supplement description of Schizocrania ? Helderhergia, sp. nov. 
