132 PALEONTOLOGY 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 Scmzo- 

 crania. There are sjDecies 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 strise. 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 iSchizoo-ania ? Helderb&rgia, sp. nov. 



