128 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



From the structure of the cardinalia with its moder- 

 ately divergent brachiophore plates, strengthened by 

 fulcral plates, it is to be deduced that Mendacella is 

 more closely related to Wattsella and Parmorthis than 

 to Dalmanella and its allies. 



Genus IDIORTHIS McLearn 1924 



PI. 21, figs. 34-38 



McLearn, Geol. Surv. Canada, Mem. 137, 1924, p. 56, 

 pi. 3, figs. 19-24, pi. 4, figs. 1-4, pl. 28, fig. 10. 



Genoholotype. — /. matura McLearn 1924. 

 Description. Exterior. — Suborbicular, hinge- 

 line narrow, cardinal extremities rounded ; lateral pro- 

 file piano- to concavo-convex, the dorsal valve usually 

 convex at the umbo but flat or concave in front of the 

 middle. Anterior commissure slightly sulcate ; ventral 

 interarea the longer, curved, apsacline ; beak incurved ; 

 delthyrium open. Dorsal interarea short, anacline. 

 Ornamentation multicostellate. Test punctate. 



Ventral interior. — Delthyrial cavity deep; teeth 

 strong; dental plates thick, divergent, continued as 

 ridges around the periphery of the diductor scars. 

 Muscle field cordate, occupying nearly half the length 

 of the valve. Diductor scars large, irregularly sub- 

 triangular, longer than the adductor track which is 

 elongate, rectangular; adductor scars elliptical; ad- 

 juster scars not visible, or very narrow and situated at 

 the base of the dental plates. 



Dorsal interior. — Cardinalia ponderous, brachio- 

 phores and their supporting plates not separable, sub- 

 parallel or slightly divergent. Cardinal process tri- 

 lobed, its shaft obscured by the median ridge which is 

 thick and clavate posteriorly but tapers to a point 

 toward the front and terminates at the margin of, or 

 just in front of, the anterior end of the muscle field. 

 Muscle area reduced by the encroachment of the crural 

 base supports which have forced the posterior adduc- 

 tors "up on the truncated anterior face of the crural 

 lamella, which, expanding laterally, curve around the 

 postero-lateral borders of the greatly enlarged and 

 almost circular anterior adductor scars" (McLearn). 

 Geologic range. — Early and } Middle Silurian. 



American Species 



Idiorthis ovila McLearn 1924 

 /. matura McLearn 1924 



European Species 

 ? Orthis eJgelliana (Salter MS.) Davidson (Sil. Brach., 

 p. 228) 



Distinguishing characters. — These are the ex- 

 tremely large ventral muscle area, strong divergent 

 dental plates, subparallel arrangement of the brachio- 

 phore supports, large size and structure of the median 



ridge and cardinal process, and nature of the dorsal 

 musculature. These characters are combined with a 

 concavo-convex exterior. 



Discussion. — Idiorthis most closely resembles 

 Wattsella in the internal structure of the valves. The 

 ventral muscle field is obcordate as in the former 

 genus, with a rather wide adductor track. In the 

 dorsal valve the brachiophore plates, situated on each 

 side of the median ridge, are subparallel. The sockets 

 appear to be defined by small socket-plates. The 

 encroachment of the adductor muscles upon the front 

 ends of the brachiophore supports in the type specimens 

 is due to their being gerontic individuals. This fact 

 also accounts for the peculiar structure of the cardinal 

 process and the median ridge. Accordingly it is our 

 view that Idiorthis as now understood is an old-age 

 stage of Wattsella, but in the absence of young speci- 

 mens this can not be proved. 



Idiorthis has not been identified outside of the Arisaig 

 section but McLearn has suggested a resemblance of 

 Orthis edgelliana Salter to its genotype. 



Genus PARMORTHIS Schuchert and Cooper 

 1931 



(Gr. farmey shield) 



PL 21, figs. 1-16, 29 



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

 p. 246. 



Genoholotype. — Orthis elegantula Dalman 

 1828, K. Svensk. Vet. Akad. HandL f. 1827, p. 117, 

 pl. 2, fig. 6. 



Description. Exterior. — Suborbicular, elongate 

 elliptical or shield-shaped in outline; margins convex, 

 front margin frequently slightly produced; hinge- 

 line narrower than the greatest shell width; cardinal 

 extremities rounded; lateral profile plano-convex, ven- 

 tral valve ventricose, dorsal valve with a shallow sul- 

 cus; ventral interarea strongly curved, slightly apsa- 

 cline, umbo swollen, beak strongly curved; dorsal 

 interarea short, anacline, beak nearly obsolete; del- 

 thyrium usually open, notothyrium closed by the car- 

 dinal process. Multicostellate, rarely fasciculate. Test 

 punctate. 



Ventral interior. — Delthyrial cavity deep; teeth 

 ponderous, crenulated on the inner surface; crural 

 fossettes deep; accessory socket large; dental plates 

 strong, vertical, subparallel or only slightly divergent; 

 muscle area confined to the delthyrial cavity; adduc- 

 tor track usually narrow, linear, slightly elevated; 

 diductor scars triangular, not extended appreciably an- 

 terior to the adductor track, covering the walls of the 

 delthyrial cavity; adjustor impressions narrow; a 

 slight median thickening extends anteriorly from the 

 adductor track, and on each side of this ridge and 



