GENERA OF THE SUBORDER ORTHOIDEA 



53 



Dorsal interior. — Notothyrial cavity shallow ; brach- 

 iophores decidedly orthoid, sharply defined, rather 

 elongate, triangular in section, obliquely placed, form- 

 ing the margin of the notothyrium, postero-ventral 

 edge carinate; cardinal process a low carina widened 

 slightly at its front. Diductor impressions on each 

 side of the cardinal process; adductor impressions small 

 pits, the posterior pair a little outside the anterior pair, 

 which are close to the median ridge. Median ridge 

 thick and wide, extending for half the length of the 

 valve or more. 



Geologic range. — Middle Cambrian of Bohemia. 

 The only known species, the genotype, was referred 

 by Walcott (1912) to BiUingsella. 



Distinguishing characters. — Orthis romingeri 

 differs so markedly from the genotype of BiUingsella 

 and all other species of that genus as to make it neces- 

 sary to set it apart as the type of a new genus. Extern- 

 ally, the species is transverse, not subquadrate like 

 BiUingsella, the ventral interarea is strongly arched, 

 and the dorsal valve nearly flat or concave. The 

 interior of the ventral valve is not strikingly different 

 from that of BiUingsella imdstromi except in detail; 

 in Bohemiella the inside pallial trunks unite in a broad 

 curve, whereas in BiUingsella they unite in a sharp V. 

 In the dorsal valve, however, there are striking and 

 important differences. The notothyrial cavity is very 

 shallow; the brachiophores are long and thin, pro- 

 jecting into the valve as in Hesferorthis. In the geno- 

 type of BiUingsella, the brachiophores are rudimentary 

 and not so well developed as in Bohemiella. The 

 median ridge in the latter is broad and thick and 

 extends a short distance in front of the middle. The 

 posterior adductors are situated a little outside the 

 anterior pair and are slightly smaller. 



Discussion. — There is an interesting dorsal valve 

 in the U. S. National Museum collection (see Walcott 

 1912, pi. 90, fig. 2k), showing old-age characters in 

 the thickening of the shell, and in the development of 

 bulbs of adventitious substance on the distal extremi- 

 ties of the brachiophores. In this specimen the pos- 

 terior part of the valve has become so thick that the 

 adductor muscles have migrated forward and occupy 

 the front half of the valve. On the inside of the free 

 end of the brachiophores the bulbs hang like dew on 

 a blade of grass, and have preserved the free ends of 

 the brachiophores so that their true length can be seen. 



Genus OLIGOMYS Schuchert and Cooper 1931 



(Gr. oligos ^^ sm&W; ?/;«; = muscle) 



PI. l,figs. 4, 8, 15, 17 



Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1 93 I, 

 p. 243. 



Genoholotype. — Orthis exforrecta Linnarsson 

 1876, Bihang till k. svensk. Vet.-Akad. Handl., vol. 3, 

 no. 12, pp. 12-13, pi. 2, figs. 13-19, pi. 3, figs. 20, 

 21, as illustrated by Walcott (1912, pi. 88, figs. 1-11). 



Description. Exterior. — Small, transversely oval 

 to subsemicircuiar, hinge-line straight; cardinal angles 

 usually obtuse; lateral profile very unequally biconvex 

 or plano-convex. Dorsal valve provided with an in- 

 conspicuous sulcus best developed at the posterior of the 

 shells and becoming obsolete toward the front; ventral 

 interarea long, curved, strongly apsaclinc ; delthyrium 

 open; dorsal interarea shorter than ventral, moder- 

 ately anacline; notothyrium open. Surface mul- 

 ticostellate to fascicostellate ; costellas covered by 

 parvicostelLx. 



Ventral interior. — Delthyrial cavity deeply sunk, 

 teeth very small, dental plates nearly obsolete ; a strong 

 thickening along each margin of the delthyrium serves 

 to strengthen the palintrope and give support to the 

 teeth. Muscle tracks deeply sunk; diductors sub- 

 triangular in outline; adductor track linear, widening 

 toward the front and extended forward beyond the 

 ends of the diductors for a short distance; pallial marks 

 consisting of two strong divergent trunks extending 

 forward for more than half the length of the valve. 



Dorsal interior. — Brachiophores short, widely diver- 

 gent, sockets shallow; cardinal process simple, thick- 

 ened, and widened anteriorly; a strong wide median 

 elevation extends the length of the valve, corresponding 

 to the external sulcus; anterior adductor scars the 

 larger, and separated from the posterior adductors by 

 a thick curved ridge. Diductor scars visible on each 

 side of the cardinal process. 



Geologic range. — Middle Cambrian of Norway 

 and Sweden and Wales. The only species are: 

 Dillingsella exforrecta (Linnarsson) 1876 

 B. hicksi (Salter MS.) Davidson 1868 

 B. rugosicoslala Walcott 1905 



Discussion. — This genus is distinguished from Bill- 

 ingsella by the very unequal convexity and external 

 shape of the valves and by its internal structure. 

 According to Walcott, the species here referred to 

 Oligomys are provided with a short concave imper- 

 forate deltidium "like that of Conchidium nysius Hall 

 and Whitfield," extending about one-fifth the length 

 of the delthyrium. If such a structure actually exists 

 it would constitute an important difference between 

 this genus and BilUtigsella, but it was not observed by 

 US; if, however, it is actually present, it must then be 

 compared with the pedicle spoon found in Protorthis. 



The ventral muscle field of Oligomys is small and 

 tripartite as in BiUingsella ; it is usually somewhat ele- 

 vated on a deposit of adventitious shell at the bottom 

 of the very deep delthyrial cavity. The adductor track 

 is narrow and linear, extended noticeably in front of 

 the diductors, the marks of which arc visible on the 

 sides of the dental plates and outside the adductor track. 

 The latter is usually separated from the diductors by 

 narrow longitudinal ridges. 



The dental plates are different from those of BiU- 

 ingsella in being convergent toward the bottom of the 

 valve. Whether there is an actual union of these 



