138 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Heterorthls is known from only one species in 

 America but is better represented in continental 

 Europe and Great Britain, where there are four forms. 

 Bancroft^"® refers Orthts 'patera Salter (Davidson, 

 Brit. Foss. Brach., vol. 3, pi. 30, figs. 1-8) to this 

 genus but the musculature and pallial markings here 

 are not those of Heterorthls. The species may belong 

 to a new group, but is related to the heterorthids by 

 the structure of the cardinalia and the general physi- 

 ognomy of the shell. Heterorthls appears to be closely 

 related to the members of the Harknessellinse in the 

 structure of the cardinalia and the musculature. 



In the Bohemian fauna there is a shell figured by 

 Barrande^"^ which has the musculature of the ventral 

 valve of Heterorthls. These two figures are referred 

 to a new species which we name H. barrandet. Orthls 

 Inclyta Barrande also has widely divergent, semiflabel- 

 late diductor scars and is placed likewise in Heterorthls. 

 In this species the adjusters are considerably more 

 developed than in H. alternata and more than is usual 

 in H. clytie. 



Ventral interior. — Delthyrial cavity shallow; teeth 

 strong; dental plates short, widely divergent, con- 

 tinued as low ridges along the lateral margins of the 

 muscle area. Muscle field subpentagonal, usually 

 bilobed in front. Diductor scars elongate, usually ex- 

 tended in front of the anterior end of the adductor 

 scar. Adjuster scars narrow, located at the base of 

 the dental plates. Adductor scar elongate rectangular. 

 Pallial markings obscure. 



Dorsal Interior. — Cardinalia heterorthoid, notothy- 

 rial cavity occupied by the cardinal process; brachio- 

 phores bladelike, cemented to the valve by adventitious 

 shell deposited on their inside surface and dorsal edge. 

 Cardinal process lobed, usually so overgrown as to 

 appear simple; dorsal muscle impressions indistinct. 



Geologic range. — Upper Ordovician of Europe, 

 with the following species: 



Orthis vesfertilio Sowerby 1839 

 Harknessella jonesi Bancroft 1928 

 H.suiflicataBincToh 1928 

 H . sui>guadrata Bancioh 1928 



Subfamily HARKNESSELLIN^ Bancroft 



Heterorthidae having biconvex valves, coarser orna- 

 mentation than is usual in the Heterorthinje, a promi- 

 nent fold on the ventral valve and an equally promi- 

 nent sulcus in the dorsal. Embraces the following 

 genera: 



Harknessella Reed 



Reuschella Bancroft 



Stneathenella Bancroft 



Genus HARKNESSELLA Reed 1917 

 PI. 20, figs. 6-10 



Reed, Trans. Roy. Sec. Edinburgh, vol. 51, pt. 4, 1917, 

 p. 862, pi. 11, figs. 3-7. 



Genoholotype. — Orthls vespertlllo Sowerby 1839, 

 Sil. Syst., pi. 20, fig. 11. 



Description. Exterior. — Shell generally small or 

 medium in size, usually subquadrate in outline; hinge- 

 line straight; cardinal extremities acute or more rarely 

 obtuse; lateral profile unequally biconvex, the dorsal 

 valve usually having the greater convexity; anterior 

 commissure sulcate; sulcus deep, ventral fold usually 

 low but prominent. Ventral interarea longer than 

 the dorsal, apsacline; umbo gently convex. Dorsal 

 interarea prominent, moderately anacline, umbo gently 

 convex. Surface costellate to fascicostellate. Shell 

 punctate. 



"" Mem. and Proc. Manchester Lit. .ind Philos. Soc, 

 vol. 72, 1928, p. 69. 



'"'Syst. Sil. Boheme, vol. 5, pt. 1, pi. 66, fig. II, 11a 

 and I 4. 



Distinguishing characters. — Harknessella is 

 distinguished by the contour and profile of the valves, 

 the subquadrate form being rather unusual in punctate 

 shells. In the ventral interior the muscle-scars are 

 dalmanelloid and in the dorsal the cardinalia suggest 

 those of Heterorthls. This structural ensemble, com- 

 bined with a fascicostellate exterior, makes a combina- 

 tion unique among the Dalmanellacea. The external 

 form is similar to that of Carinlferella of the Upper 

 Devonian. 



Discussion. — The greatest morphologic interest is 

 in the dorsal valve and in the cardinalia. As stated 

 above, the greatest similarity is with Heterorthls. In 

 Harknessella the brachiophores are rather long, extend- 

 ing dorso-ventrally. They are unsupported except for 

 adventitious shell deposited on the inside and along the 

 dorsal edge, which unites them to the valve. This is 

 similar to the condition in Heterorthls, but in Harkness- 

 ella there is a much greater development of the noto- 

 thyrial platform. This is usually swollen about the 

 inside surfaces of the brachiophores, and in some in- 

 stances the platform is nearly flush with the ventral 

 edge of the brachiophores. In front of the notothyrial 

 platform are two deep indentations separated by the 

 median ridge; it is in these pits that the posterior 

 adductor muscles were lodged. The cardinal process 

 is distinctly lobed as in all Dalmanellacea but the shaft 

 may be so swollen as to hide or obscure the original 

 lobation. In one undescribed species the cardinal 

 process is delicate, with a slender shaft but an expanded 

 and lobed myophore. 



The distinction between Harknessella and Smeath- 

 enella and Reuschella is rather difficult to see if one 

 deals with either the actual shell or a wax replica 

 thereof, but in internal molds is not so troublesome. 



