212 PALEONTOLOGY OF NEW YORK. 



The species comprising this group have a very close external resemblance to 

 the resupinate shells included under Groups IV, V and VI. There are still 

 some differences. In these the hinge-line is shorter, the shells often proportion- 

 ally narrower, the cardinal area of the pedicle-valve less elevated ; the orna- 

 mentation of the surface is striate but much more finely ; the striae are hollow, 

 tubulose and produced into short spines,* as in Rhipidomella. 



The interior presents an arrangement of the muscular scars very similar to 

 that seen in 0. sinuata and its allies. In the pedicle-valve there is a short 

 subquadrate or obcordate area with thickened, elevated margins, and deeply 

 sunk in the substance of the shell ; in the opposite valve a less distinctly 

 defined, quadripartite area. In Schizophoria, however, the adductor impres- 

 sion occupying the summit of the median ridge dividing the muscular area 

 of the pedicle-valve, is more distinct, the hinge-teeth are more divergent 

 and less ponderous ; in the brachial valve the crural plates are much less 

 divergent and more erect; the cardinal process, which in young shells, has 

 much the same character as in Rhipidomella, becomes absorbed and thus nar- 

 rowed with age making a thin and sharp ridge ; concomitant with this change 

 is the formation, in the delthyrial cavity, of one, two or even three minor ridges 

 on each side of the original process, so that in old shells the posterior face of 

 the process appears to be multilobate In the very convex brachial valve, four 

 (rarely six) deep pallial sinuses take their origin at the anterior margin of the 

 muscular area, passing forward as broad, simple, subparallel bands, to near the 

 margin of the valve, where they bifurcate and become arborescent. To these 

 differences must be added one of distinctive importance, viz., the abundantly 

 punctate character of the shell-structure. 



In America the earliest representative of the group is the species here 

 described as Schizophoria seneda from the Clinton fauna, this type of structure 

 being thus coeval with that of Rhipidomella. In the Lower Helderberg fauna 

 is Orlhis multislriata, Hall, which is followed in the Corniferous limestone by 

 0. propinqua. Hall, in the Hamilton by O. Tulliensis, Hall, and in the later Devo- 

 nian by O. lowensis, Hall, 0. impressa, Hall, 0. Macfarlanii, Meek, O. Tioga, Hall, 



*See Davidson, British Carboiiifei-ons Brafihioi)oda, pi. xxx, tig. \d. 



