GENERA OF THE SUBORDER ORTHOIDEA 



127 



are quite distinct. There are also notable differences 

 between Rrsscrella and Wattsflla, particularly in the 

 ribbing characters. We are, however, willing at pres- 

 ent to accept Mr. Bancroft's statement regarding the 

 relationships of these two genera in the absence of 

 specimens of immature Resserella. 



Genus HORDERLEYELLA Bancroft 1928 



PI. 22, figs. 30, 32-35 



Bancroft, Mem. Proc. Manchester Lit. Philos. Soc., vol. 72, 

 1928, p. 178, pl. 1, figs. 15-18. 



Genoholotype. — H. fUcata Bancroft 1928. 



Description. Exterior. — Shells rather small, sub- 

 semielliptical or subquadrate in outline; hinge-line 

 straight, cardinal extremities obtuse. Lateral profile 

 subequally biconvex. Anterior commissure sulcate, sul- 

 cus shallow; ventral fold subcarinate. Interareas sub- 

 equal, ventral one apsacline, dorsal anacline. Ventral 

 umbo rather strongly convex, dorsal umbo gently 

 convex. Surface strongly fascicostellate. Test prob- 

 ably punctate. 



Ventral interior. — Delthyrial cavity rather deep; 

 dental plates short and strong; muscle area short, sub- 

 pentagonal; diductor impressions subtriangular. Ad- 

 ductor impression linear, occupying a depression formed 

 by the posterior extension of the fold. Pallial impres- 

 sions unknown. 



Dorsal interior. — Notothyrial cavity shallow, brach- 

 iophores long, brachiophore plates convergent dorsally 

 and uniting with the median ridge. Sockets defined 

 by fulcral plates. 



Geologic range. — Ordovician of England and 

 Wales. The species are : H. flicnta Bancroft, H. n. sp. 



Discussion. — This genus was originally placed by 

 its author in the Harknessellinae, but it appears to us 

 to belong nearer Wattsella, since its brachiophores are 

 supported by convergent plates and there is a fulcral 

 plate bounding the socket. From Wattsella it differs 

 in the greater convergence of the brachiophore sup- 

 ports, which do not appear as parallel slits in internal 

 molds. 



Genus MENDACELLA Cooper 1930 



(Lat. mendax, lying) 



PI. 22, figs. 1-5, 8 



Cooper, Jour. Pal., vol. 4, 1930, pp. 377, 380, pl. 36, 

 figs. 2, 16-18 (uteris), also figs. 6, 9 {mullochiensis). 



Genoholotype. — Orthis uheris Billings 1866, 

 Cat. Sil. Foss. Anticosti, p. 42. 



Description. Exterior. — Similar to Rhifidomella 

 but with the valves subequally convex. The ventral 

 valve is always the larger of the two. 



Ventral interior. — Delthyrial cavity moderately 

 deep; teeth large; dental plates strong, subparallel or 

 divergent, continued forward as ridges on the outer 

 margins of the diductor scars. Muscle area bilobed in 

 front; diductor scars long and narrow; adductor 

 ridge wide or narrow, carinatc or flat, never extended 

 to the front of the diductor scars. Adjuster scars 

 small or large, narrow. Pallial markings not observed. 



Dorsal interior. — Cardinalia confined ; brachio- 

 phores stout, bluntly pointed, slightly divergent; sock- 

 ets deep, defined by a small concave fulcral plate. 

 Cardinal process small, shaft slender, not extending 

 anterior to the ends of the brachiophore plates, myo- 

 phore expanded lobate; a low broad median ridge 

 extending from the ends of the brachiophore plates 

 nearly to the middle of the valve. Adductor scars 

 subequal, separated in some species by low ridges at 

 right angles to the median ridge. 



Geologic range. — Upper Ordovician and early 

 Silurian. 



American Species 



Orthis tequivalvis Shaler 1865 ^ O. uheris Billings 1866 

 Rhifidomella rhynchonellijormis (Shaler) 1865 

 Schizofhorella arisaigensis McLearn 1924 



? Rhifidomella circulus (Hall) 1843 

 ? R. lenticularis Foerste 1903 

 ? R. tenuilineata Foerste 1913 



European Species 



Orthis minuscula Barrande 



O. mullochiensis Davidson (Sil. Brach., p. 221) 



Distinguishing characters. — This genus is dis- 

 tinguished especially by the Dalmanella-Wke. character 

 of the interior and the Rhipidomella-Vke exterior. Ex- 

 ternally, however, it differs from Rhipidoinella in hav- 

 ing the ventral valve usually the larger. Internally the 

 variations of the muscle-scars, dental plates, and car- 

 dinalia from those of Rhipidomella are striking. The 

 muscle area does not have the broadly semiflabellate 

 diductor scars enclosing the oval adductor ones. The 

 diductor impressions of Mendacella are quite direct and 

 narrow, and ordinarily only slightly divergent; they 

 have a rather strong resemblance to the same scars in 

 Dalmanella. 



Discussion. — There is some variation in the ventral 

 musculature between the various species placed by us 

 in this genus. In M. uheris from the Ellis Bay forma- 

 tion (late Ordovician) the diductors are frequently 

 separated by a low wide ridge, but in the same species 

 from the Silurian Gun River formation the ends of the 

 diductors are separated from each other by a very 

 narrow ridge only. The other Silurian species such as 

 M. arisaigensis and M. mullochiensis all resemble the 

 Silurian M. uheris in this respect. There is also a vari- 

 able development of the adjustor scars within the same 

 species and in different species. 



