92 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



forming a fold in the young shell. This costa con- 

 tinues elevated for some distance in the mature shell, 

 but toward the front a sulcus is developed to correspond 

 to the elevation of the dorsal median fascicle. 



Genus PTYCHOPLEURELLA Schuchert and 

 Cooper 1931 



(Gr. ^iMx, wrinkle; pleura, rib) 



PI. 6, figs. 1-6, 9, 32, 33 



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

 p. 244. 



Genoholotype. — Orthis bouchardi Davidson 

 1847, London Geol. Jour., Feb. 1847, p. 64, pi. 13, 

 figs. 5-8. 



Description. Exterior. — Shells small, subellipti- 

 cal, ventral valve subpyramidal ; anterior margin emar- 

 ginate, lateral margins convex; hinge-line wide and 

 straight; cardinal extremities obtuse; lateral profile 

 unequally biconvex, the dorsal valve with the greater 

 convexity; anterior commissure sulcate ; ventral inter- 

 area plane or slightly curved, strongly apsacline to pro- 

 cline, delthyrium open or constricted by lateral plates; 

 dorsal interarea much shorter than the ventral, strongly 

 curved, faintly apsacline to orthocline, notothyrium 

 open, umbo convex, beak fairly well defined, incurved ; 

 usually a strong sulcus in the dorsal valve, not uncom- 

 monly also in the ventral. 



Ornamentation multicostate, but the ribs are very 

 sparse, sharp, and elevated. The whole surface is cov- 

 ered by imbrices which are occasionally produced into 

 short hollow processes. Shell substance impunctate. 



Ventral interior. — Delthyrial cavity moderately 

 deep; teeth small, sharp; fossettes oblique; lateral 

 plates overhanging the delthyrial cavity; dental plates 

 nearly obsolete in old shells, muscle area cordate; ad- 

 ductor track elongate, expanded in front, commonly 

 elevated; diductor tracks and scars elongate, narrow; 

 adjustor scars not clearly visible; a small pedicle callist 

 in the apex; pallial and ovarian marks obscure but the 

 traces are similar to those of Glyftorthis. 



Dorsal interior. — Brachiophores forming the margin 

 of the notothyrial cavity, supported by thick shelving 

 deposits that define the notothyrial platform; cardinal 

 process a linear ridge, with the diductor impressions on 

 its sides; a median ridge extending more than half the 

 length of the shell. Adductor scars small. 



Geologic range. — Middle Ordovician to late 

 Silurian. 



American Species 



Orthis (?) lamellosa Twenhofel 1914 

 O. rugiflicala Hall and Whitfield 1872 

 Skenidium (?) nodocostatum Rowley 1904 

 Glyftorthis sublamellosa Cooper 1930 

 Ptychofleurella matafedia, n. sp. 



European Species 



Orthis bouchardi Davidson 1 847 

 O. keisleyensis Reed 

 O.lafworthi Davidson 1883 

 O. ? monticula Salter^" 



Discussion. — This genus comprises small neat shells 

 and is unique for certain external and internal features. 

 Externally it is characterized by the subpyramidal ven- 

 tral valve and rather strongly convex dorsal valve, 

 which in all of the species is marked by a deep sulcus 

 bounded by two costs elevated considerably above 

 their fellows. Inside these two primary costae there 

 are always implanted two secondary ribs near or far 

 from the former according to the species. On the 

 ventral valve there is always a median costa, more or 

 less elevated at the beak but becoming depressed in the 

 shallow sinus toward the front. The sinus is bounded 

 by the two main lateral ribs on each side of the pri- 

 mary costa, and toward the front secondary ones are 

 intercalated. 



In the ventral interior the muscle field is elevated, 

 forming a pseudospondylium. The plan of the muscles 

 is essentially the same as that of Glyftorthis except that 

 the diductor scars are not so drawn out in front and 

 the adjustor scars are not so well developed. In front 

 of the adductor track there is a remnant of a median 

 ridge as in Orthis s. s., Hesfer orthis, and Glyftorthis. 

 Pallial impressions have not been observed in their 

 entirety and their reduction is a variation from Glyft- 

 orthis. The brachiophores and cardinal process are 

 orthoid, and the muscle field is small, quadripartite. 

 The posterior adductor scars are very small, and almost 

 obsolete in P. bouchardi and P. rugiflicata. 



According to Davidson,^* the test of O. bouchardi 

 is punctate, but the writers were unable to find any 

 punctae. The constriction of the delthyrium by lateral 

 plates is a feature of the ventral valve worthy of note, 

 and has been seen in several species, especially P. lamel- 

 losa and P. subla77iellosa. This reduction of the delthy- 

 rium has been carried to such a degree in P. lamellosa 

 that it is but a narrow slit. This tendency of shell 

 growth along the margins of the delthyrium has been 

 noticed in several genera, i. e., Schi'zoramma (S. got- 

 landica), Orthis s. s., Hesferorthis, etc., and is more 

 fully discussed on page 23. TwenhofeP^ says of 

 P. lamellosa: 



Wetting of the ventral area shows that narrow side plates 

 are annexed to the sides of the foramen; these are supposed 

 to be continuous with the teeth, as in O. bouchardi, the 

 nearest related species. These plates simulate deltidial 

 plates, with which, however, they are probably in no way 

 homologous. 



=^° Reed, F. R. C, Pal. Indica, ser. XV, vol. 7, no. 2, 

 1912, pi. 5, figs. 1, 2. 



"Brit. Foss. Brach., vol. 3, 1866-1871, p. 210. 



" Geol. Surv. Canada, Mem. 1 54, 1927 (1928), p. 176. 



