BRACHIOPODA. . 209 



1552. Orthis, Whitpibld. Bull. No. 3, American Mas. Nat. Hist., p. 45, pi. vi, figs. 1-5. 



1882. Orthis, Hall. Eleventh Rept. State Geologist of Indiana, p. 285, pi. xxi, figs. 18-25. 



1553. OrtUs, Hall. Twelfth Rept. State Geologist of Indiana, p. 324, pi. xxix, figs. 1-5. 



1883. Orthis, Hall. Rept. N. Y. State Geologist for 1882, pi. xxxvi, figs. 1-16, 19-21 and pi. xxxvii, 



figs. 1-4. 

 1883. Orthis, White. Twelfth Rept. U. S. Geol. Suvv. Terr., p. 164, pi. xli, fig. 4. 

 1885. Orthis, Foekste. Bull. Denison University, p. S3, pi. xiii, fig. 10. 

 1887. Rhipidomys, (Ehlert. Fischer's Manual de Conchyliologie ; Brachiopodes, p. 1288. 

 18S8. Orthis, Hbrkick. Bull. Denison University, vol. iii, p. 38, pi. v, fig. 9. 

 1889. Orthis, Netteleoth. Kentucky Fossil Shells, p. 39, pi. xxxii, figs. 32-35 ; pp. 40, 45, pi. xvi, 



figs. 4-6, 12-14, 23, 24 and pi. xvii, figs. 33-35. 

 1889. Orthis, Simp.son. American Philosophical Soc., p. 437, fig. 1. 



1889. Orthis, Beechek and Clarke. Mem. N. Y. State Museum, p. 17, pi. i, figs. 13-18. 



1890. Rhipidomella, CEhlbkt. Joui'nal de Conchyliologie. 



This is a large and very compact group of orthids, fundamentally character- 

 ized by the large flabelliform muscular scars of the pedicle-valve, but having in 

 association with this a number of other peculiarities of importance. The 

 shells are subcircular in outline, biconvex and sublenticular, with the brachial 

 valve somewhat the deeper. The hinge-line is short, the 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 hol- 

 low, often opening upon the surface; these openings probably representing 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; and 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 ; some- 

 times the former 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 



