88 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



European Species 



S. gotlandica, n. sp. 



Orthis cdligramma subflicata Reed 1917 



O. rigida Davidson 1 847 



O. {Plectorthis) subfissicosta Reed 1917 



Distinguishing characters. — The external ex- 

 pression of Schizoramma is essentially that of Hesferor- 

 this in the contour and profile of the valves and the 

 strongly apsacline, plane interarea. It differs, how- 

 ever, in having costellje instead of costs, which in some 

 species are bundled into fascicles. From Dolerorthh 

 it differs in not having a convexo-concave profile. 

 Externally Schizoramma is also marked by the pres- 

 ence of a distinct sulcus in the dorsal valve, but in some 

 species this is present only in young specimens. In old 

 shells the sulcus develops into a fold anterior to the 

 middle of the valve, as may be seen in S. gotlandica, 

 n. sp. 



Discussion. — In the United States, the species of 

 Schizoramma have been variously referred to Hehert- 

 ella and Orthostrofhia, and the genus was described 

 by Foerste as a subgroup of Hebertella. There is, 

 however, no relationship between Schizoramma and 

 Hebertella and the kinship between the former and 

 Orthostrofhia is probably also remote. The plan of 

 the ventral musculature and the structure of the car- 

 dinalia preclude the placing of Schizoramma with 

 Hebertella. The genus finds its best place in the group 

 with Hesferorthis, since it is but a modified or aberrant 

 form of that genus. Other internal resemblances with 

 Hesferorthis are seen in the plan of the musculature; 

 in the narrow, obscure median ridge; and the kidney- 

 shaped ovarian marks. Inside the dorsal valve the like- 

 ness to Hesferorthis is not so close, and Schizoramma 

 differs in its modified cardinal process, smaller adduc- 

 tors, and shorter brachiophores. 



An interesting feature of the dorsal valve of Schizo- 

 ramma is the accessory ridges on the notothyrial 

 platform diverging from the cardinal process. These 

 have been observed in other genera but are of 

 sporadic occurrence among impunctate orthids (in a 

 species of Hebertella). Such ridges are, however, quite 

 commonly developed in Schizofhoria, since in two 

 specimens showing them the cardinal process is reduced 

 to a mere septum, and this may mean that these ridges 

 function for the attachment of muscles in the absence 

 of a large cardinal process. 



Schizoramma differs from Orthostrofhia in not hav- 

 ing the convexo-concave profile, the very confined ven- 

 tral musculature, or the small, elevated muscle field 

 of the dorsal valve. Further, the ventral pallial mark- 

 ings of Orthostrofhia art more complex, more defi- 

 nitely defined, and more like those of Mimella than of 

 Schizoramma. 



It will be noticed that the writers have referred sev- 

 eral European species to this genus, and most of these 

 have been so placed from the literature; S. rigida, 



however, is represented by good specimens in the 

 Schuchert Collection and clearly belongs in this genus. 

 There may be some doubt about S. gotlandica, n. sp., 

 which differs from all other forms of the genus in being 

 transversely semicircular rather than subquadrate. An 

 undescribed transverse species, very close to S. got- 

 landica, is represented by a specimen from the Niagaran 

 of Savannah, Illinois. 



Schizoramma gotlandica, n. sp. 

 PI. 5, figs. 3, 6 



Differs from Orthis rustica Sowerby, to which it 

 has usually been referred, in its smaller size, in being 

 more strongly apsachne, and in having a more gently 

 curved interarea and a nearly flat dorsal valve which 

 is rather strongly convex. 



Geologic range. — Silurian of Gotland, Sweden. 



Cotypes in Schuchert Collection, Yale University 

 (Cat. No. 228). 



Genus DOLERORTHIS Schuchert and Cooper 



1931 



(Gr. doleros, deceptive) 



PI. 5, figs. 10, 12, 15, 17-24 



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

 p. 244. 



Genoholotype. — Orthis interflicata Foerste 1909, 

 Bull. Sci. Lab. Denison Univ., vol. 14, p. 76, pi. 3, 

 fig. 44. 



Description. Exterior. — Transversely semiellip- 

 tical to semicircular, hinge-line wide and straight, 

 cardinal extremities acute or obtuse, lateral profile 

 convexo-concave to unequally biconvex, lateral com- 

 missure flexed slightly dorsad; anterior commissure 

 broadly uniplicate. Ventral interarea long, faintly 

 curved, strongly apsacline; beak slightly incurved; 

 umbo gently convex. Dorsal interarea shorter than 

 ventral, plane, varying in position between faintly apsa- 

 cline and moderately anacline, umbo gently convex; 

 delthyrium and notothyrium open. Ornamentation 

 pauci- to multicostate. Growth filae strong in the 

 interspaces. Test fibrous, impunctate. 



Ventral interior. — Delthyrial cavity moderately deep ; 

 teeth small; crural fossettes small, oblique; dental 

 plates moderately strong in young shells but nearly 

 obsolete in adults or old specimens, extended as a 

 thickening about the muscle area in some species; 

 muscle area obovate; adductor scars elongate, form- 

 ing together a lanceolate track; diductor impressions 

 large, subcrescentic; adjustor scars not distinguishable; 

 in most species a faint ridge or septum separates the 

 pallial trunks that branch abruptly near the front mar- 

 gin of the shell. Large subreniform ovarian impres- 

 sions occupy the lateral spaces and the umbonal cavi- 

 ties. Anterior margin costate, each costa being cleft 

 centrally. 



