78 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Distinguishing characters. — Nicolella is char- 

 acterized externally by its strong angular or subangular 

 costse, which are simple or may have secondary inter- 

 polated costae in the interspaces; the piano- or concavo- 

 convex profile; the alate or subalate hinge-line; and 

 the chilidial plates that partially close the notothyrium. 



Internally it is very similar to Orthis s. s. in the ven- 

 tral musculature and in the development of the dental 

 plates. As in most shells with curved, short palin- 

 tropes, the fronts of the dental plates extend nearly 

 vertically to the floor of the valve, enclosing a rather 

 deep delthyrial cavity. The muscular pattern of the 

 ventral valve is essentially the same as in Orthis s. s. 

 The diductor-adjustor scars are triangular in outline 

 and the position of the adductor muscles is represented 

 by a narrow linear track. Pallial markings were not 

 impressed in any of the specimens studied. 



Inside the dorsal valve the most significant variation 

 from Orthis s. s. is in the shortness and bluntness of 

 the brachiophores. This variation is without doubt due 

 to the concave growth of the dorsal valve, which makes 

 the brachiophores project almost vertically in a ventral 

 direction. It is not uncommon that much adventitious 

 shell substance is deposited about the brachiophores for 

 their support. The brachiophores, because of their 

 brevity and bluntness, resemble those of Billingsella 

 to a marked degree. 



Discussion. — Wysogorsky'® gave the origin of A^. 

 actomcE (Sow.) as out of Orthis moneta Eichwald. 

 It is not, however, O. moneta of Eichwald that 

 Wysogorsky figures, but that form as interpreted by 

 Verneuil, which is undoubtedly a new species close to 

 "N . fterygoidea (Pander). It is evident that A^. 

 actonicE and the later Nicolellas are descended from 

 A^. ftery goidea and that type of shell in the early 

 Ordovician. It is doubtful, however, if Nicolella arose 

 from O. caUigramma Dalman, which Lamansky says 

 is restricted to stage B.^^ The ancestor of Nicolella 

 must be sought in the Ozarkian and will undoubtedly 

 be a billingselloid or eoorthoid form. The chilidial plates 

 of Nicolella are probable billingselloid inheritances. 



O. callactis Dalman has a very close resemblance in 

 profile and outline to Nicolella^ but differs in details 

 of ribbing, the former having a predominantly radial 

 ornamentation, costas and interspaces alike being cov- 

 ered by parvicostellse. Such fine radial ornamentation 

 has never been observed in Nicolella. 



The greatest development of Nicolella is in the 

 Baltic region of Europe and chiefly in Estonia and 

 Russia. In North America the genus is represented 

 by the single species A'^. agilera Willard. This distribu- 

 tion of Nicolella is parallel to that of Orthis s. s., which 

 is represented in North America only by O. fanderiana 

 Hall and Clarke and O. buttsi Schuchert and Cooper. 



'» Zeit. deut. geol. Gesell., vol. 52, pt. 2, 1900, pi. 8. 

 " Mem. Com. Geol., n. ser., livr. 20, 1905, p. 176. 



Genus GLOSSORTHIS Opik 1930 



PI. 4, figs. 2, 7-9, 12, 28; also figs. 3, 6, 10 



bpik. Acta at Comm. Univ. Tartuensis, A, vol. XVII, pt. 1, 

 1930, p. 82, pi. 3, figs. 26-33, t. fig. 9. 



Genoholotype. — G. tacens Opik 1930. 



Description. Exterior. — Subquadrate in outline; 

 hinge-line straight, narrower than the widest part or 

 forming the widest part of the shell; cardinal extremi- 

 ties acute or obtuse. Lateral profile unequally bicon- 

 vex, ventral valve with the greater convexity. An- 

 terior commissure rectimarginate. Ventral palintrope 

 the longer, faintly apsacline or orthocline, curved, beak 

 strongly incurved; delthyrium open. Dorsal inter- 

 area short, plane, anacline; beak scarcely defined; 

 notothyrium open. Surface costate, costas angular, 

 rarely bifurcate; interspaces marked by elevated 

 growth-lines or filje which may or may not be inter- 

 rupted so as to appear like granules. Test fibrous, 

 impunctate. 



Ventral interior. — Delthyrial cavity deep, with mar- 

 ginal thickenings; teeth ponderous, curved, placed 

 laterally of the delthyrial margins. Crural fossettes 

 deep. Dental plates sharply defined, slightly diver- 

 gent, attached directly to the floor of the valve. 

 Umbonal cavities deep. Muscular area elongate; ad- 

 ductor field linear, elevated, and prolonged into a 

 tongue-like process that extends in front of the diduc- 

 tor scars. The latter are narrow, elongate, attached 

 also on the sides of the dental plates. Adjustor scars 

 not observed. The front margin is crenulated as in 

 other Orthidae by short ribs corresponding to the ex- 

 ternal striae (interspaces), each internal rib bearing a 

 groove. 



Dorsal interior. — Notothyrial cavity deep; brach- 

 iophores orthoid, blunt, rather short. Cardinal process 

 a linear ridge thickened on its ventral edge. Adventi- 

 tious shell forming a prominent notothyrial platform 

 and supporting the brachiophores. Muscle area con- 

 fined; posterior adductor impressions short; anterior 

 pair the larger, each divisible into an inner and outer 

 element as in Productorthis. Median ridge extending 

 to the front margin of the adductor field. 



Geologic range. — Middle Ordovician (Walchow, 

 Bo, to Kuckers, C2) of Europe. 



Species 



GlossorlMs linJa Opik 1930 



G. Linda virgata Opik 1930 



G. tacens Opik 1930 



Orthis externa Verneuil non Pander'* 



'* The species referred to is figured in Geology of Russia, 

 vol. 2, pi. 13, fig. l!a-c. This has nothing to do with 

 Orthis extensa (Pander), which belongs to Panderina. 



