GENERA OF THE SUBORDER ORTHOIDEA 



139 



In HarknessflLi (see pi. 20, fig^. 8, 9) the base of the 

 brachiophores and their supporting tissue are triangular 

 in plan, with the antero-lateral extremity of the tri- 

 angle somewhat drawn out. The adductor pits are 

 represented by two posteriorly directed acute lobes on 

 each side of a median depression. Other differences 

 are discussed farther on. 



Harktu-sscUa is not uncommon in the Middle Ordo- 

 vician (Caradocian) of the British Isles but is so far 

 unknown in North America. 



Subgenus REUSCHELLA Bancroft 1928 

 PI. 20, figs. 11-15 



Bancroft, Mem. and Proc. Manchester Lit. and Philos. 

 Soc., vol. 72, 1928, p. 180, pi. 2, figs. 9-12, t. fig. 5. 



Genoholotype. — R. semiglohata Bancroft 1928. 

 Description. Exterior. — Shells commonly rather 

 large, subquadrate in outline, hinge-line straight, car- 

 dinal extremities acute; lateral profile unequally bicon- 

 vex or convexo-concave, the dorsal valve having the 

 greater convexity. Anterior commissure more or less 

 strongly sulcate; dorsal sulcus deep in young stages, 

 obsolete or prominent in old stages; ventral fold sub- 

 carinate. Ventral interarea the longer, strongly apsa- 

 cline ; dorsal interarea anacline or orthocline. Umbos 

 gently convex. Surface costellate to fascicostellate. 

 Shell punctate. 



Ventral interior. — Delthyrial cavity shallow; teeth 

 strong; dental plates stout, flaring, rather short, con- 

 tinued as ridges on the lateral margins of the muscle 

 area. Muscle field pentagonal in outline, gently 

 bilobed in front. Adductor field linear, strongly im- 

 pressed; diductor scars elongate, tear-shaped. Adjus- 

 ter scars rather large. Pallial markings unknown. 



Dorsal interior. — Notothyrial cavity obsolete, as it 

 is filled completely by the notothyrial platform and the 

 cardinal process; brachiophores bladelike as in Heter- 

 orthis, cemented to the valve in front by adventitious 

 substance. Sockets deep; cardinal process with a 

 lobate myophore, shaft slender or swollen. Adductor 

 impressions occupying troughs bounding the median 

 ridge. 



Geologic range. — Middle Ordovician (Caradoc- 

 ian-post-Harnage) of the British Isles, with the follow- 

 ing species: 



Reuse hella bilobata (Sowerby) 1839 

 R. horderleyensis Bancroft 1928 

 R. semiglobataBancioh 1928 



Discussion. — Reuschella differs from Harknessella 

 in its coarser ornamentation and in details of the car- 

 dinalia. Bancroft's description of this genus was made 

 entirely from internal molds, and the distinctions from 

 Harknessella that he points out and figures appear very 

 slight. From specimens kindly sent to us by Bancroft 

 we could discern no fundamental differences in the 

 cardinalia. We recommend therefore that Bancroft's 

 genus be considered a subgenus of Harknessella. 



Genus SMEATHENELLA Bancroft 1928 

 PI. 20, figs. 1-5 



B.incroft, Mem. and Proc. Manchester Lit. and Philos. 

 Soc., vol. 72, 1928, p. 177, pi. 2, figs. 1-5. 



Genoholotype. — S. hamagensis Bancroft 1928. 



Description. Exterior. — Shell rather large, sub- 

 quadrate in outline; hinge-line straight; cardinal ex- 

 tremities usually obtuse; lateral profile unequally 

 biconvex or convexo-concave, the dorsal valve having 

 the greater convexity. Anterior commissure sulc.ite; 

 ventral fold strongly carinate. Interareas subequal, 

 ventral interarea apsacline, dorsal anacline; umbos 

 gently convex. Surface unequally costellate as in 

 Rafinesquina. 



Ventral interior. — Delthyrial cavity confined, shal- 

 low; dental plates short and stout, muscle area rhom- 

 boidal in outline. 



Dorsal interior. — Like Harknessella; notothyrial 

 cavity closed by the cardinal process and notothyrial 

 platform. Brachiophore plates slender, heterorthoid. 

 Sockets deep, median elevation corresponding to ex- 

 ternal large sulcus. 



Geologic range. — Middle Ordovician (Carodoc- 

 ian) of the British Isles, with the single species S. har- 

 nagensis Bancroft. 



Distinguishing characters. — Smeathenella is 

 very closely allied to Harknessella but differs in its 

 ornamentation, lateral profile, and in details of the 

 cardinalia. The ornamentation has been described by 

 Bancroft as "markedly Rafinesquinoid," that is, there 

 are coarser costells separated by many fine costellas. 

 This ribbing, in connection with a much less convex 

 dorsal valve than is usual in Harknessella or Reuschella, 

 will serve to distinguish the genus. Other differences 

 setting apart this genus from the other two are to be 

 found in the interior of both valves. The dental 

 plates are usually proportionally shorter and the mus- 

 cle field is smaller. In the dorsal valve the cardinalia 

 are more slender and the adductor pits not as deeply 

 excavated. 



Family SCHIZOPHORIID^ Schuchert 1929 



Progressive and terminal Dalmanellacea, probably 

 derived out of the Dalmanellidas,^"* having biconvex 

 or lenticular valves in the older formations and in the 

 younger growth stages, but in mature and old forms 

 with the dorsal valve usually larger than the ventral. 

 Surface ornamented by costellse, often hollow. In the 



'"* PionoJema, earliest of the Schizophoriidx, appears 

 simultaneously with the DalmancUidx. It is therefore diffi- 

 cult at present to state with assurance what stock gave rise 

 to Schizofhoria and allies. 



