100 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



process comes to resemble that of Plectorthis, hence 

 this is no serious objection to placing Austinella near 

 D'lnorthis. The brachiophores appear to be of the 

 Dinorthis type and are supported chiefly by adventitious 

 shell substance. 



When Foerste named the genus in 1909 he stated 

 in the introductory sentence that "Dinorthis scovillei 

 belongs to a group of species typified by Orthis kanka- 

 kensis, McChesney and including also Orthis whit- 

 fieldi, N. H. Winchell." This statement implied, if 

 it did not definitely say, that O. kankakensis is the 

 genotype of Austinella. Evidently Bassler so inter- 

 preted Foerste's words, because O. kankakensis is desig- 

 nated as the type in the Ordovician-Silurian Index, p. 

 1002.*- Later, however, Foerste ''^ stated that "Aus- 

 tinella scovillei is regarded as the genotype of the pro- 

 posed group, Austinella." As we have already noted, 

 Foerste selected the genotype in 1909 and this selection 

 must stand. 



Genus PAL.ffiOSTROPHOMENA Holtedahl 

 1916 



Holtedahl, Videns. Skrift., I. Mat.-Naturv. Klasse, 1915, 

 no. 12, 1916, p. 43, pi. 7, figs. 1, 2. 



Genolectotype (Opik). — P. concava Holtedahl 

 1916 (non Strofhomena concava Schmidt; see Opik, 

 Acta et Comment. Univ. Tartuensis, A, vol. 17, pt. 1, 

 1930, p. 57). 



Original description. — General form as in Strofho- 

 mena and Strofhonella but differs distinctly in interior 

 characters of dorsal valve, the cardinal process being not 

 bilobed but as in the Orthida: simple, having the shape of 

 a short vertical plate, posteriorly united with the crural 

 plates, which end rather abruptly. The cardinal process is 

 supported by a faint median elevation in the valve stretch- 

 ing for some distance anteriorly. Interior of ventral valve 

 not known. 



Geologic range. — Lower Ordovician (Stage 3) 

 of Norway. The only species is the genotype. 



DisctJssiON. — According to Holtedahl, this genus 

 has a dorsal interior which clearly relates it to the 

 orthids, but the exterior is exactly like that of Strofho- 

 mena. This combination of characters is suggestive of 

 V alcourea but according to the figures the specimens 

 are more finely ornamented than in the latter. Fur- 

 thermore, Palteostrofho?nena is provided with concen- 

 tric wrinkles which are unknown in Valcourea. Al- 

 though the exterior is very close to that of Strofhotnena 

 it would seem best to consider this shell as an orthoid 

 homoeomorph of that genus. The simple cardinal 

 process is unknown in the Strophomenacea (except in 

 the Plectambonitidse, which may be aberrant orthids), 



"^U. S.Nat. Mas., BuU. 92, 1915. 



«' Bull. Denison Univ., vol. 17, 1912, p. 130. 



so that it seems more reasonable to consider Palrrostro- 

 fhomena an orthid and to place it tentatively near 

 V alcourea. 



Opik (1930) says that Holtedahl did not have 

 before him specimens of Schmidt's Strofhomena con- 

 cava, but a Norwegian shell that he identified with 

 that species. Accordingly Schmidt's form is not the 

 genotype, as stated by Schuchert and Levene (1929), 

 but Holtedahl's Pala-ostrofhotnena concava becomes 

 the type. 



Genus PLANIDORSA Schuchert and Cooper 

 1931 



(Lat. planus, flat; dorsion, back) 



PI. 16, figs. 22, 24, 25, 27; t. fig. 19 



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

 p. 244. 



Genoholotype. — p. bella Schuchert and Cooper 

 1931. 





Fig. 19. — Diagrams showing the chilidial plates and ven- 

 tral musculature of Planidorsa. 



Description. Exterior. — Shells rafinesquinoid, 

 lateral margins concave ; hinge-line wide ; cardinal 

 angles acute; lateral profile piano- to slightly concavo- 

 convex; lateral commissure straight; anterior com- 

 missure rectimarginate ; fold and sulcus faintly devel- 

 oped in young shells; cardinal extremities set oiT by 

 oblique sulci; ventral interarea longer than the dorsal, 

 curved, slightly apsacline, beak strongly incurved, del- 

 thyrium open, umbo swollen ; dorsal interarea plane, 

 strongly anacline, notothyrium partially covered by 

 chilidial plates and partially closed by the cardinal 

 process also, beak undefined; ornamentation multi- 



