Report of the State Geologist. 587 



Rhipidomella, (Khlert. 18 

 Synonym; Rhipidorrvys, (Ehlert. L887. 

 XI. Group of Orthi^ Mjohelini, Leveille. 

 Plate 12, Figs. 1-13.) 



Shell subcircular in outline, biconvex and sublenticular, with 

 the brachial valve somewhat the deeper Binge-line short. 

 cardinal area narrow, especially on the brachial valve. The 

 surface bears a slight median depression on each valve, and is 

 covered with fine, rounded, subequal striae which are hollow, 

 often opening upon the surface ; these openings probably repre- 

 sent the broken bases of short tubular spines. 



The pedicle valve bears two strong diverging teeth, planted 

 firmly upon the valve at the bottom of the delthyrium, and 

 extending upward and outward at their extremities ; from their 

 bases a more or less strongly defined curving ridge extends 

 forward, bordering the muscular area. The muscular area 

 extends from one-third to five-sixths the length of the valve and 

 is deeply impressed ; the pedicle-scar fills the entire rostral cavity ; 

 the adductors occupy a small central scar which is completely 

 enveloped by the great diductors. A median ridge arises in 

 front of the adductors, dividing the diductors ; sometimes the latter 

 scars rest upon a general anterior flattening of this ridge, and 

 in rare instances the ridge divides the separate components of 

 the adductor impression. The margin of the entire muscular 

 area is thickened .and elevated, and outside of this are deeply 

 pitted ovarian markings. 



In the brachial valve the dental sockets are deep and narrow, 

 the crural plates extremely prominent, sometimes supporting 

 short crura. The cardinal process is erect, strongly arched on 

 its anterior face, often very thick and greatly elevated ; the edge 

 of its posterior face is multilobate, the posterior surface itself 

 having a trilobed appearance. The muscular area is quadrupli- 

 cate, comparatively small and usually indistinct. A broad, low, 

 median ridge extends forward from the base of the cardinal 

 process. The shell-structure is coarsely fibrous and very strongly 

 punctate, the perforations being large and generally more 

 abundant along the furrows between the striae. 

 (Upper Silurian — Upper Carboniferous.) 



139 



