GENERA OF THE SUBORDER ORTHOIDEA 



115 



The deltidium of Vellamo appears to be formed in 

 precisely the same manner as that of Clitamhonites, 

 but at maturity the apical foramen is not sealed as in 

 the latter genus though it is closed in old-age specimens. 



Clittimbomtfs appeared first geologically (Kunda), 

 and died out with the Middle Ordovician. Vellamo, 

 on the other hand, appeared later (Chazy) and en- 

 dured through the rest of the Ordovician. 



Genus CLINAMBON. n. gen. 



(Gr. klino, bend, and amhon, umbo) 



PI. 8, figs. 24, 28 



Genoholotype. — Anomites anomala Schlotheim 

 1822, Nachtnige, p. 65, pi. 14, fig. 2. 



Description. Exterior. — Subquadrate in outline, 

 valves very unequal, the ventral one pyramidal, the 

 dorsal flat and elongate. Anterior commissure unipli- 

 cate. Ventral interarea strongly procline, longer 

 than the dorsal one. Interarea of the dorsal valve 

 unusually long; chilidium wide and strongly arched. 

 Deltidium perforate in young shells but foramen sealed 

 in old ones. Valves multicostellate as in Vellamo. 



Ventral interior. — There is a prominent spondylium, 

 the sides of which are infolded at the front, giving a 

 pinched-in effect. The median septum is short. 



Dorsal interior. — Exactly as in Vellamo except that 

 the structures are greatly overdeveloped. The cardinal 

 process is fused to the ventral border of the ponderous 

 chilidium. Median ridge short. Notothyrial platform 

 ponderous and lateral thickenings greater than in any 

 other member of the Clitambonitidas. Brachiophores 

 flat blades clearly visible near the sockets. 



Geologic range. — Middle Ordovician (Jewe and 

 Kegel, Do) of Estonia. The genotype is the only 

 known species. 



Discussion. — The unusual development of the 

 dorsal interarea and the ponderous chilidium are fea- 

 tures which set this shell aside from Vellamo. The 

 ornamentation is like that of the latter genus, and the 

 appearance in the geologic column comes long after 

 the disappearance of Clitamhonites, indicating relation- 

 ships with Vellamo rather than any other group of the 

 Clitambonitinx. 



Genoholotype. — Orthisina marginata Pahlen 

 1877, Mem. Acad. Imp. Sci. St. Petersh., 7th ser., 

 vol. 24, no. 8, p. 33, fig. 8, pi. 3, figs. 11-15, pi. 4, 

 figs. 1-3. 



Description. Exterior. — Sub-strophomenoid to 

 hebertelloid in lateral profile and outline; ventral 

 interarea longer than dorsal, usually strongly apsa- 

 cline. Deltidium having an apical foramen; chi- 

 lidium complete. Surface multicostellate. Test fibrous, 

 impunctate. 



Ventral interior. — Delthyrial cavity deep; teeth 

 large; spondylium supported at the front by a promi- 

 nent median septum and laterally by low, very short 

 septa. Muscle attachments confined to the spondy- 

 lium. Adductor and diductor tracks linear. Sub- 

 peripheral rim low, but prominent. 



Dorsal interior. — Notothyrial cavity shallow ; noto- 

 thyrial platform swollen, with prominent thick lateral 

 extensions which support the brachiophores. Median 

 ridge strong; brachiophores orthoid; cardinal process 

 simple, cemented to the under surface of the chilidium. 

 Subperipheral rim prominent; adductor field large, 

 with the muscle attachments commonly elevated. 



Geologic range. — Middle Ordovician (Kuckers) 

 of Europe. 



Species 



Orthisina marginafd Pahlen 1877 

 O.f'yro/! Eichwald 18+0 

 O.volborthi Pahlen 1877 

 Gonambrinltes marginatus asfer Opik 1930 

 G. marginatus magnus Opik 1930 

 G. fanderi'd'^W. 1930 

 Hemifronites carrickensis Reed 1917 

 H. thomsoni Reed 1917 



? G. infiexa Pander 1830 



Discussion. — Estlandia differs from Gonambonites, 

 which it resembles and with which it has usually been 

 associated, in having an apical foramen, median septa 

 in both valves, and a complete chilidium. The orna- 

 mentation of Estlandia and its internal structure, how- 

 ever, suggest relationship with Vellamo. As in the 

 Orthidas, Dinorthid.x, and other groups, it is reason- 

 able to expect a convexo-concave stage of development. 

 Estlandia appears, then, to be a convexo-concave stage 

 of Vellamo, of rather short geological range and local 

 development. 



Genus ESTLANDIA Schuchert and Cooper 

 1931 



PI. 8, figs. 6, 8, 9; t. fig. 10 



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

 p. 245. 



Gonambonites (pars) Opik, Acta et Comm. Univ. Tartu- 

 ensis. A, vol. 17, 1930, pp. 222-227, pi. 19, figs. 229- 

 236, 248; pi. 20, figs. 236-239; t. figs. 24, 25. 



Genus HEMIFRONITES Pander 1830 

 Pi. 8, figs. 10-15 



Pander, Beitr. Gcogn. Russ. Reiches, 1830, pp. 71, 74, 

 pi. 3, fig. 14, pi. 18, fig. 6, pi. 28, fig. 22. 



Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, p. 238, 

 figs. 9, 10. 



Genolectotype (Dall). — H. tumida Pander 

 1830, 



