GENERA OF THE SUBORDER ORTHOIDEA 



123 



Description. — This genus externally resembles 

 Rhipuiomi-lltt and Th'u-nicUa hut has a well marked 

 fold on the ventral valve and a sulcus on the dorsal 

 which are not reversed in the young stages. Internally 

 the arrangement of the ventral muscle-scars is similar 

 to that of Rhipuiotiiflla in the imprisonment of the 

 adductor field by the diductor scars. The diductor 

 impressions are never broadly flabellate as in Rh'tpi- 

 domi-lla and the adjustor marks are usually clearly 

 visible as in CannijereUti. Furthermore, the diductor 

 impressions are separated by a low ridge which is 

 forked much as in Cnriniferrlta. This is a feature 

 never shown by Th'tetnflla or Rh'tfuiomrlla in which 

 the median ridge is always direct and unforked. 



In the dorsal valve the cardinalia are ponderous and 

 Strongly resemble those of Cnriniferelln and Rhifi- 

 domdla but are totally unlike those of Thicmella which 

 are delicate and confined. In our classification Aulac- 

 ella is placed in association with Dalmnnella and 

 C artntjerella because of the close similarity of the 

 ventral musculature and dorsal cardinalia. 



The only known species is the German Orthis 

 eifelensis. 



Genus PROSCHIZOPHORIA Maillieux 1911 



PI. 19, figs. 25, 26, 30, 32 



Maillieux, Bull. Soc. Beige Geol., Pal., d'HydroL, vol. 25, 

 1911, p. 177, pi. B, fig. I. 



Genoholotvpe. — Orthis fersonata Zeiler 1857, 

 Verb. Nat. Hist. Ver. Bonn, vol. 14, p. 48, pi. 4, 

 figs. 9-11, emend. Kayser 1892, Jahrb. k. Preuss. 

 Geol. Landesanst., for 1890, p. 98, pi. 11, figs. 3-5, 

 pi. 12, figs. 1-4. 



Description. Exterior. — Like Schizophorin, shell 

 large and thick. 



Ventral interior. — Delthyrial cavity deep; teeth 

 large; dental plates thick and strong, continued as 

 ridges for some distance around the margins of the 

 diductor scars; muscle area large, cordate, wider than 

 long; diductor scars large, divergent, not enveloping 

 the adductor scars in front; adductor impressions 

 forming two semielliptical impressions, together making 

 an oval scar; the adjustor scars elongate marks on 

 the outside of the diductor impressions. 



Dorsal interior. — Cardinalia ponderous; brachio- 

 phores as in Carinijerella, divergent; sockets deep; 

 shaft of cardinal process a stout linear ridge, rounded 

 ventrally and continued forward nearly to the anterior 

 ends of the adductor scars; adductor impressions sub- 

 equal, separated from each other by low ridges at right 

 angles to the median ridge. 



Geologic range. — Lower Devonian (Coblenz- 

 ian) the only known species being the European 

 P. personata (Zeiler). 



Distinguishing characters. — This genus diflFers 

 from Schizophoria, which it most closely resembles, by 



the cordate ventral muscle area. The ventral adduc- 

 tor ridge so prominent in Schizophoria is lacking here. 

 In the dorsal valve the adductor scars are subequal in 

 size and separated by a horizontal ridge at right angles 

 to the median elevation. The prominent cardinal 

 process without expanded myophorc is another signifi- 

 cant variation from Schizophoria. 



Discussion. — The name Proschizophoria was not 

 happily chosen, as this genus is neither the earliest of 

 the Schizophorias nor is it the progenitor of the group. 

 The structure of the dorsal valve obviously relates it to 

 Carinijerella and the Devonian representatives of the 

 Dalmanellidx. Neaverson*" places Orthis provulvaria 

 Maurer in this genus, but its dorsal valve has the 

 musculature and pallial markings of Schizophoria, 



Genus LEVENEA Schuchert and Cooper 1931 



PL 18, figs. 19-23,25-32; t. fig. 14 



Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, 

 p. 246. 



Genoholotvpe. — Orthis subcarinata Hall 1857, 

 10th Rept. N. Y. State Cab., p. 43, figs. 1, 2. 



Description. Exterior. — Subquadrate to subcir- 

 cular in outline, hinge-line narrower than the greatest 

 shell width ; cardinal extremities rounded ; lateral pro- 

 file unequally biconvex to plano-convex; ventral inter- 

 area longer than dorsal, apsacline; delthyrium open. 

 Notothyrium closed by the cardinal process. Surface 

 multicostellate. Test fibrous, punctate. 



Ventral interior. — Delthyrial cavity deep; teeth 

 strong, located at the angle of the delthyrial and hinge 

 margins. Crural fossettes deep. Dental plates strong 

 in the young, obsolete in old forms, thickened on the 

 inside to form the fossette. Muscle field pentagonal 

 in outline ; diductor tracks deeply impressed, elongate ; 

 adductor track wide, elevated, shorter than the diduc- 

 tor marks. Adjustor tracks located on the sides of the 

 dental plates, strongly impressed. Pallial markings as 

 in Isorthis and Schizophoria. 



Dorsal interior. — Brachiophore plates blades like 

 those of Dalmanella ; cardinal process prominent, shaft 

 short or obsolete, myophore lobate; adductor field sub- 

 circular or widely elliptical; adductor scars subequal. 

 Pallial markings similar to those of Isorthis (see t. 

 fig. 14). 



Geologic range. — Silurian to Middle Devonian. 



American Species 



Orthis lenticularis Vanuxein 1 842 



O. quadrans Hall 1861 



O. subcarinata H.1II 1857 



Dalmanella macra Dunbar 1920 



D. subcarinata Dunbar 1919 (non Hall) 



? Orthis concinna Hall 1859 

 ? O. solilaria Hall I860 



"" Stratigraphical Palaeontology, 1928, p. 256. 



