GENERA OF THE SUBORDER PENTAMEROIDEA 



155 



Family CLARKELLID/E Schuchert and 

 Cooper 1931 



Syntrophiacea, externally like Syntroph'ta and with 

 a sf)ondylium simplex or a pseudospondylium. In the 

 dorsal valve the brachiophore supports are divergent, 

 discrete plates. Ventral pallial markings as in 

 Billingselln. 



Geologic range. — Middle Cambrian to Lower 

 Ordovician. 



Embraces the following genera: 



Syntrophioides Schuchert and Cooper 

 Syntrofhitut Ulrich 

 Clarkella Walcott 

 Yangtzeella Kolarova 



Genus SYNTROPHIOIDES Schuchert and 

 Cooper 1931 



PI. 15, figs. 20, 23 



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

 p. 247. 



Genoholotype. — Billingsella harlanensis Walcott 

 1905, Proc. U. S. Nat. Mus., vol. 28, p. 236; Camb. 

 Brach., 1912, p. 746, pi. 87, figs. 5-5d (as Wimanella 

 harlanensis) . 



Description. Exterior. — Subelliptical to subsemi- 

 elliptical, hinge-line straight; cardinal extremities usu- 

 ally obtusely angular; lateral profile biconvex; an- 

 terior commissure uniplicate; dorsal fold not greatly 

 pronounced. Ventral interarea long, apsacline, del- 

 thyrium open ; surface marked only by fine, concentric 

 lines of growth. Test probably fibrous, impunctate. 



Ventral interior. — Delthyrial cavity rather shallow, 

 dental plates in the type specimens not sharply marked ; 

 musculature clearly billingselloid, having a prominent 

 central adductor track which widens toward the front 

 and is cleft into halves by a low ridge; the front of 

 the adductor track elevated slightly. Diductor tracks 

 smaller than the adductor ones, rounded in front, 

 separated from the latter by low ridges. From the 

 anterior ends of the diductors two pairs of pallial 

 sinuses extend in an antero-lateral direction nearly 

 to the front border as in Billingsella. 



Dorsal interior. — Notothyrial cavity shallow, brach- 

 iophore plates convergent and resting on the floor of 

 the valve ( ? ) ; musculature prominent, posterior ad- 

 ductors larger than the anterior pair, suboval in out- 

 line; anterior pair about half the size of the posterior 

 one. Two pallial sinuses have their origin at the point 

 of contact with the posterior adductors in the mid-line 

 of the shell and are separated by a slight median eleva- 

 tion. They extend in this closely appressed condition 

 as far as the front margins of the anterior adductors. 



where they branch abruptly toward the antero-lateral 

 margins. 



Geologic range. — Middle Cambrian of Tennes- 

 see with Billingsella (or Wimanella) harlanensis Wal- 

 cott and ? B. (or W. ) saffordi Walcott. 



Discussion. — This genus is readily separated from 

 Billingsella by its concentrically marked surface, syn- 

 trophiid form, and dorsal musculature. B. harlanensis 

 was placed in Wimanella by Walcott because of its 

 smooth shell, but since that genus appears to have a 

 costellate surface, the species under consideration can 

 not be left there. 



The dorsal musculature of Syntrophioides is well 

 defined and strongly suggests that seen in Syntrophina 

 and Clarkella, representing a divergence probably out 

 of some ancestor of the Billingsellidas of the early 

 Middle Cambrian. In these circumstances we see how 

 the slight generic differences in these early Cambrian 

 forms may have given rise to later independent families 

 and even superfamilies. 



Genus SYNTROPHINA Ulrich 1928 



PI. 15, figs. 1, 2, 15-19, 30,31 



Ulrich in Weller and St. Clair, Missouri Bur. Geol. Mines, 

 2d ser., vol. 22, 1928, p. 74 (without description). 



Genoholotype. — Syntrophia campbelli Walcott 

 1912, Camb. Brach., pp. 801-802, t. figs. 73A-F. 



Description. Exterior. — Subelliptical, hinge-line 

 narrower than the greatest width of the shell ; cardinal 

 extremities rounded. Lateral profile strongly bicon- 

 vex; ventral valve deeply sulcata, dorsal valve with a 

 prominent fold; anterior commissure uniplicate. Ven- 

 tral palintrope short, apsacline; delthyrium open. 

 Dorsal interarea shorter than the ventral, apsacline to 

 anacline; notothyrium open. Surface marked only 

 by fine concentric lines of growth. Test fibrous, 

 impunctate. 



Ventral interior. — Delthyrial cavity deep, teeth 

 strong, dental plates convergent, forming a true spon- 

 dylium simplex. Muscle impressions borne on the 

 upper surface of the spondylium; diductor marks on 

 the sides and floor of the spondylium at the back; 

 adductor impressions (?) in front of the diductor scars 

 at the narrow end of the spondylium. Pedicle callist 

 at the posterior. Pallial marks two broad trunks ex- 

 tending antero-laterally from beneath the spondylium. 

 Umbonal cavities and umbo-lateral spaces marked by 

 low radial ridges. Complete or incomplete accessory 

 septa occur in the vicinity of the spondylium in some 

 specimens and show a convergence toward Clarkella. 



Dorsal interior. — Notothyrial cavity deep, brachio- 

 phores long, brachiophore supports divergent, extend- 

 ing to the floor of the valve. Cardinal process rudi- 

 mentary, elevated on a narrow shelf at the posterior 

 of the notothyrial cavity. Posterior adductor scars the 



