130 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Genoholotype. — Orthis gervillei Defrance 1827, 

 Diet. Sci. Nat., vol. 51, p. 152. 



Description. Exterior. — Dalmanelloid, subcircu- 

 lar to subquadrate; lateral profile piano- to slightly 

 concavo-convex; anterior commissure rectimarginate 

 or sulcate ; sulcus on the dorsal valve shallow ; ven- 

 tral interarea curved, beak incurved; dorsal interarea 

 anacline; ornamentation strongly fascicostellate, the 

 chief distinguishing feature of the genus. 



Ventral and dorsal interior. — Essentially the same 

 as in Orthis elegantula Dalman (^PartTiorthis). 



Geologic range. — Upper Silurian ? to Lower 

 Devonian of Europe, with the following species: 



Orthis dorsoflicata Beclard 



O. gervillei Defrance 



O. sedgwicki D'Archiac and Verneuil 



Discussion. — Fascicostella is distinguished from all 

 other Dalmanellacea by its peculiar ornamentation, 

 since it has the greatest development of fasciculation 

 seen in any orthid. Internally the species most closely 

 resemble Parmorthis elegantula of Gotland. How- 

 ever, the hinge-line is proportionately wider and the 

 cardinal extremities less rounded and the ventral valve 

 never so ventricose. The ventral musculature is like 

 that of P. elegantula, but has larger adjustor scars. 

 The pallial sinuses are also like those of the Gotland 

 species in being closely together, subparallel, and sepa- 

 rated by a narrow, low ridge. The anatomy of the 

 dorsal valve agrees closely with that of P. elegantula. 



The close similarity of the internal anatomy of Fasci- 

 costella to that of Parmorthis elegantula suggests that 

 the genus had its origin in that form. Fascicostella 

 seems to be localized in the Lower Devonian of 

 Europe, except for Kozlowski's report** of "Dalman- 

 ella" gervillei in the highest Silurian of Poland 

 (Borszczow). 



Family BILOBITID^E Schuchert and Cooper 

 1931 



Aberrant, specialized Dalmanellacea originating in 

 the Dalmanellidas, having an emarginate anterior mar- 

 gin, narrow hinge-line, and a deep sulcus in each valve. 



The only genus is Bilobites Linnasus, extending 

 from the Upper Ordovician into the early Middle 

 Devonian of Europe and North America. 



Genus BILOBITES Linnaeus 1775 



Linna:us, Syst. Nat., ed. Miiller, vol. 6, 177S, p. 325. 

 Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, p. 204, 



pi. 5B, figs. 11-14. 

 Syn. Diccelosia King 1850. 



Genoholotype. — Anomia biloha Linnaeus 1767, 

 Syst. Nat., 12th ed., p. 1154. 



Description. Exterior. — Small, bilobed, anterior 

 margin emarginate, hinge-line narrower than the 

 greatest width of the shell; lateral profile unequally 

 biconvex; anterior commissure broadly sulcate. Ven- 

 tral interarea the longer, curved, apsacline, umbo 

 swollen, sulcate, beak incurved; dorsal interarea short, 

 anacline, beak projecting slightly, incurved; surface 

 multicostellate ; shell coarsely punctate. 



Ventral interior. — Umbonal cavity deep ; teeth 

 strong; dental plates thick and rather obscure; muscle 

 area bilobed, thickened along the margins; divided 

 centrally by a sharp ridge corresponding to the external 

 sulcus; diductor scars semiflabellate, divergent, not 

 enveloping the adductor impressions; adductor scars 

 borne on the central ridge, obscure; adjustor scars 

 obscure. 



Dorsal interior. — Cardinalia thick, brachiophores 

 ponderous, long, bladelike, widely divergent; sockets 

 shallow; cardinal process thick; myophore bilobate, 

 crenulated on its posterior face, its shaft extended for- 

 ward and merging with the median ridge formed by 

 the impression of the external sulcus on the inside of 

 the shell. 



Geologic range. — Upper Ordovician (White- 

 house group of Girvan; Richmondian of Gaspe, 

 Quebec); widely in the Silurian; Lower Devonian 

 (New Scotland, United States) and early Middle 

 Devonian (Bohemia, Ggi). 



American Species 



Bilobites acutilobus (Ringueberg) 1888 

 B.bilobus (Linna:us) 1767 

 B. indentus Cooper 1930 

 B. varicus {Cor\r3iA) 1838 



European Species 



Bilobites bilobtts (Linnasus) 1767 

 B. verneuilianus Lindstrom 

 Orthis dimera Barrande 



Discussion. — External form most readily distin- 

 guishes Bilobites from all other genera of the Dal- 

 manellacea. Internally the ventral musculature and 

 cardinal process are decidedly dalmanelloid. The 

 brachiophores appear to be modified dalmanelloid, 

 bladelike, and suggest an origin out of Dalmanella. 

 Beecher®" has figured a deltidium perforated at the 

 apex in young Bilobites, but such a structure has not 

 been observed in any of the mature forms studied or 

 in any other punctate shell. If this actually exists only 

 in the young of Bilobites, it is one more crumb of evi- 

 dence that a delthyrium covered by a deltidium is 

 actually the primitive condition in early brachiopod 



Pal. Polonica, vol. 1, 1929, p. 70. 



'Studies in Evolution, 1901, p. 402. 



