GENERA OF THE SUBORDER ORTHOIDEA 



133 



Family RHIPIDOMELLID^ Schuchcrt 

 1913, emended 



(=partim Rhipidomellinse Schuchert 1913) 



Subcircular, progressive, and terminal Dnlmanella- 

 cea with broad flabellate diductor scars, completely 

 enclosing the elliptical adductor field in front. Brach- 

 iophores short, without fulcral plates. 



This large family begins in the early Silurian and 

 dies out in the late Permian. It embraces the follow- 

 ing genera: 



RhipiHomcUa Oehlert 

 Perditocard'tnia Schuchert and Cooper 

 Plijtyorthis Schuchert and Cooper 

 Thiemella W^illiams 



11. 



The genetic relations seem to be as shown in Table 



Table 11 



Thiemella 



Platyorthis 



Rhipldomella- 



-> Perditocardinia 



Dalmanella (D. tersa ?) 



Genus RHIPIDOMELLA Oehlert 1890 

 PI. 19, fig. 3; pi. 20, figs. 22-24, 26, 27 



Oehlert, Jour. Conch. (3), vol. 30 [38], 1890, p. 372. 

 Horn. Rhifidomys Oehlert 1887. 



Genoholotype. — Terehratula mtchelim L'Eveille 

 1835, Mem. Soc. Geol. France, vol. 2, p. 39, pi. 2, 

 figs. 14-17. 



Description. Exterior. — Subtrigonal to circular, 

 anterior margin not uncommonly emarginate, hinge- 

 line narrow; lateral profile unequally biconvex, the 

 brachial valve having the greater convexity, with the 

 ventral one concave at the front in many species; an- 

 terior commissure faintly uniplicate or rectimarginate ; 

 in some instances a sulcus on each valve; ventral 

 interarea the longer, curved, apsacline, umbo swollen 

 or gently convex, beak incurved; dorsal interarea 

 greatly reduced, ortho- to apsacline; delthyrium open, 



notothyrium usually closed by the cardinal process or 

 partially by chih'dial plates; surface multicostellate, 

 hollow costcUx- numerous. Fibrous, punctate. 



Ventral intrrior. — Delthyrial cavity shallow; dental 

 plates abbreviated, teeth strong, divergent, elongate; 

 a broadly curved ridge extending from the bases of the 

 low dental plates around the margin of the muscle 

 field; muscle field large, flabellate, not confined to the 

 delthyrial cavity, occupying from one-third to five- 

 sixths the length of the valve and usually deeply im- 

 pressed; diductor scars semiflabcllate, separated from 

 each other by a sharp or low broad ridge, completely 

 enclosing the adductor scars, which form an elliptical 

 patch just anterior to the pedicle callist; adjustor scar 

 commonly discernible on the outside of the diductor 

 scar; pedicle callist occupying the delthyrial cavity. 



Dorsal interior. — Cardinalia confined, sockets wide, 

 deep, without concave fulcral plates; brachiophores 

 long, bluntly pointed, supported by adventitious sub- 

 stance deposited beneath their anterior edge; sharp 

 processes or points on the ends of the brachiophores 

 have been interpreted as crura; cardinal process large, 

 myophore commonly ponderous, lobatc; shaft short. 

 Median ridge extending to the middle of the shell. 

 Muscle field quadripartite, the posterior scars the 

 larger. Ovarian and pallial impressions occupying the 

 area of the shell not covered by the muscle marks. 



Geologic range. — Silurian (Clinton) to close of 

 Permian. 



American Species 



Rhifidomella aha (Hall) 1863 

 R. altiroslris Mather 1915 

 R. arkansana Girty 1911 

 R.assimilis (Hall) 1859 

 R. burlingtonensis (HiW) 1858 

 ^. <rario«arM (Swallow) 1858 

 R. clarkensis (?i\vi[\aw) 1863 

 R.cleobi! (HaU) 1863 

 R.cumberlanJia (HaW) 1859 

 R.dalyana (Miller) 1881 

 R. diminutiva Rowley 1900 

 R. discus (Hall) 1859 

 R.dubia (Hall) 1858 

 R. ellstvorthi Tinsey 1922 

 R.emarginata (HaU) 1859 

 R. gooduini {'^enahoih) 1889 

 R.hessensis}L\ng \9i\ 



R. hybrid a (Sowerby) 1839 (now a composite species, in- 

 cluding forms referable to Mendacella) 

 R.idonea (Hall) 1867 

 R.intermedui SuuScT 1918 

 R. jersey ensis Wcller 1914 

 R. leAuejuetiana Chrke 1907 

 R. teonard ensis King 1931 

 R.leucosia (Hall) 1860 

 R.lkia (Billings) 1860 

 R.loganiCUxVc 1907 

 R.lucia (Billings) 1874 

 R. magnicardinalis Focrste 1 909 



