130 PALEONTOLOGY OF NEW YORK. 



cle-furrow, abruptly intercepting the ornamentation but not penetrating the 

 substance of the shell, begins just below and behind the apex, extends over a 

 greater or less portion of the radius of the valve, and, at its distal end, is 

 produced into a short tubular sipho, which traverses the substance of the shell 

 obliquely backward, emerging on the interior surface, where it produces a 

 narrow groove, and usually terminates before reaching the margin of the valve. 

 On the interior, the position of the external groove is marked by a thickened 

 ridge extending from the apex, and this is continuous with the thickened mar- 

 gins of the internal groove, which, in advanced age, may become so developed 

 as to envelop this groove, except at its outer end. 



The larger or brachial valve is depressed-conical, with the apex more 

 strongly directed backward than in the opposite valve. The interior shows a 

 fine longitudinal ridge or septum extending from the apex forward. Other- 

 wise the internal markings are not satisfactorily known. 



Shell-substance composed of alternating lamellae of corneous and mineral 

 matter, the latter often removed in fossilization, making the shell appear essen- 

 tially phosphatic. Surface ornamentation usually consisting of fine, crowded 

 or distant, sometimes lamellose concentric lines, occasionally crossed by radiat- 

 ing lines or ridges. 



Type, Orbicula Morrisi, Davidson. Wenlock limestone. 



Under the above limitations these fossils appear to constitute a very compact 

 generic group subject to no essential variation. In order to properly interpret 

 their pedicle-characters as usually preserved, emphasis must again be laid upon 

 the fact, that, by compression of the tenuous shell, the external and internal 

 features of the groove are frequently made to appear as continuous, and the 

 careful observer will seek examples in which the normal contour of the shell 

 has not been disturbed in fossilization. In some species the external depres- 

 sion of the pedicle-area is very considerable (Z>. Newherryi, D. Conradi, D. ampla), 

 though not of the extent seen in Discinisca, nor so greatly thickened ; and 

 such species often show more distinctly the composition of the base or 

 floor of the groove lying between the apex and the external aperture, a tripartite 



