GENERA OF THE SUBORDER ORTHOIDEA 



77 



Orth'ts as here defined has two groups of closely re- 

 lated shells; one, Orthis s. s., typified by O. callactis 

 with its flat or concave dorsal valve; and the other, 

 O. calligramma, being an example of the biconvex 

 type. It Is the former kind of shell that may have 

 given rise to NicoUlla. 



Genus CYRTONOTELLA Schuchert and 

 Cooper 1931 



PI. 2, fig. 14; pi. 4, figs. 1,4,5, 11 



Schuchert and Cooper, Amer. Jour. Sci. (S), vol. 22, 193 1, 

 p. 243. 



Genoholotype. — Orthis seniicirctilans (YAc\Ywa[A) 

 1829, Zool. Spec, vol. 1, p. 276, pi. 4, fig. 10. 



Description. Exterior. — Semielliptical, anterior 

 margin rounded, lateral margins straight or concave, 

 hinge-line wide and straight; cardinal angles a right 

 angle or acute. Lateral profile concavo-convex; 

 anterior commissure rectimarginate. Ventral valve 

 strongly convex, interarea short, curved, anacline, ven- 

 tral umbo strongly convex, beak curved, delthyrium 

 open, wider than long, beak frequently resorbed. Dor- 

 sal valve concave, interarea short, plane or slightly 

 curved, anacline to hypercline; notothyrium partially 

 closed by the cardinal process. Ornamentation multi- 

 costellate, interspaces with concentric growth-lines. 

 Test impunctate. 



Ventral interior. — Delthyrial cavity deep, teeth 

 small; dental plates very widely divergent, thick, 

 overhung by the palintrope, receding; muscle field 

 wider than long, arrangement of the muscles as in 

 Orthis s. s. Large pedicle callist in apex. Anterior 

 and lateral margins crenulated. 



Dorsal interior. — Notothyrial cavity shallow ; brach- 

 iophores short, widely divergent, sockets small; car- 

 dinal process a stout ridge projecting above the surface 

 of the interarea; from the notothyrial platform a low 

 ridge extends forward to the center of the shell. 

 Adductor field small, individual components not recog- 

 nizable; pallial sinuses four in number, sent oflF from 

 the anterior margins of the muscle area. One pair 

 extends antero-laterally and the other anteriorly, but 

 both die out before reaching the margins of the shell. 



Geologic range. — Middle Ordovician (Echino- 

 sphaerites limestone to Wesenberg) of Europe. 



Species 



Orthis semicircularis (Eichwald) 1829 

 O. concava Schmidt 



Discussion. — Cyrtonotella differs externally from 

 Orthis in its multicostellate exterior. The ventral 

 muscle area is much more transverse and the plates 

 less prominent as a result of the strong curvature of 

 the ventral umbo. The ventral pallial sinuses and 

 ovarian impressions are essentially the same as in 

 Orthis. In the dorsal valve the pallial markings are 



like those of O. aqualis but the cardinalia are much 

 less robust. This group is therefore separated from 

 Orthis because of the character of the ornamentation 

 and the peculiar profile of the shell. 



Genus NICOLELLA Reed 1917 



PI. 2, figs. 1-6 



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

 p. 860, pl. 10, figs. 25-27. 



Genoholotype. — Orthis actoniw Sowerby 1839, 

 Sil. Syst., p. 639, pl. 20, fig. 16. 



Description. Exterior. — Semielliptical to semi- 

 circular; hinge-line wide and straight, not uncom- 

 monly wider than the greatest width of the shell; 

 cardinal extremities acute, drawn out; lateral profile 

 piano- to concavo-convex; anterior commissure broadly 

 sulcate. Ventral interarea very short, curved, ortho- 

 cline to faintly anacline, umbo swollen, beak strongly 

 curved, delthyrium open; beak commonly resorbed. 

 Dorsal interarea nearly equal in length to the ventral 

 interarea or slightly shorter, anacline to hypercline, 

 notothyrium frequently partially covered by chilidia. 

 Ornamentation paucicostate to multicostate, costas an- 

 gular, concentric growth-lines and growth-lamellse 

 covering the entire surface. Test fibrous, impunctate. 



Ventral interior. — Delthyrial cavity moderately deep ; 

 teeth strong; crural fossettes very deep; dental plates 

 strong and defined by shallow umbonal cavities in 

 young specimens, nearly obsolete in adults; muscle 

 field subcircular; diductor scars subtriangular; ad- 

 ductor scars occupying a central linear track. 



Dorsal interior. — Notothyrial cavity obsolete; car- 

 dinalia feebly developed, greatly obscured by subse- 

 quent shell growth in adults or old individuals; brach- 

 iophores short, small and widely divergent; sockets 

 small; cardinal process lanceolate; chilidia arising be- 

 tween the margin of the brachiophores and the cardinal 

 process. Median ridge thick, extending to the middle 

 of the valve; dorsal muscle field small; pallial mark- 

 ings not observed. 



Geologic range. — Middle Ordovician (Kuckers 

 to Lyckholm) of Europe and North America. 



European Species 



N. asteroidea Reed 1917 

 N.demissa Hisinger 1837 

 N. rudis Lindstrom 

 N.sdmeO^'xk 1930 

 Orthis actonut Sowerby 1839 

 O.moneta Eichwald 1837 

 O. tt»^«<»V Sowerby 1839 

 Productus hamatus Pander 1830 

 P. fterygoideus Pander 1830 

 P.siriatus Pander 1830 



American Species 

 N.agUera VfWhid. 1928 



