SYNTROPHIID^. 



801 



Observations. — In form this species is usually most nearly related to Syntrophia calcifera 

 (Billings) (PI. CIV, figs. 1, la-i). It differs in having a less clearly defined beak and less promi- 

 nent fold on the dorsal valve. There are many points in common between Syntrophia camhria 

 and S. nundina Walcott (PL CII, figs. 4, 4a-c), but they difl'er in the shorter beak and sharper 

 mechan fold of the latter. Syntrophia camiria occurs in the Wasatch range, about 4,500 feet 

 below the base of the Ordovician, while S. calcifera and S. nundina occur in the Lower Ordovi- 

 cian. It is the oldest species of the genus and is of interest also on account of being closely 

 related in form to the Ordovician species mentioned. 



Formation and locality. — Middle Cambrian: (31c and 54o) 900 feet (274.3 m.) above the Brigham quartzite and 

 3,300 feet (1,005.8 m.) below the Upper Cambrian, in the limestone forming lb of the Ute limestone [Walcott, 1908f 

 p. 196], in Blacksmith Fork Canyon, about 10 miles (16.1 km.) ea?t of Hyrum, Cache County; (32e) the same strati- 

 graphic horizon as Locality 31c just south of the south fork of Paradise Dry Canyon {East Fork), east of Paradise, Cache 

 County; and (34m) limestone about 765 feet (233.2 m.) above the Brigham quartzite [Walcott, 1908a, p. 8], 1 mile 

 (1.6 km.) northwest of Geneva (Copenhagen), east of Brigham, Boxelder County; all in Utah. 



Syntrophia campbelli Walcott. 



Text figures 73A-F. 



Syntrophia campbelli Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, pp. 107-108, PL X, figs. 9, 9a-c. (De- 

 scribed and discussed as below as a new species. Figs. 9, 9a-c are copied in this monograph as figs. 73B 73C 

 73E, and 73F, respectively.) 



General form rotund, unequally biconvex; hinge line short. Ventral valve moderately 

 convex, exclusive of the prolonged frontal margin; it is depressed toward the front in adult 



FiGUEE 73.—SyniropUa campbelli Walcott. A, Ventral valve, the type specimen (U. S. Nat. Mus. Cat. No. 524801). B, Cast of ventral valve, 



showing cast of septum supporting the spondylium (U. S. Nat. Mus. Cat. No. 52480a). C, C, Dorsal valve (tJ. S. Nat. Mus. Cat. No. 52480b). 



D, Dorsal valve (U. S. Nat. Mus. Cat. No. 52480e). E, Section of ventral valve, showing cast of septum and spondylium (U. S. Nat. Mus. 



Cat. No. 52480c). F, Cast of dorsal valve, showing cast of spondylium (U. S. Nat. Mus. Cat. No. 524S0d). 



The specimens represented are from Locality 12q, Upper Cambrian, near Rogersville, Tennessee. Figures 73B, 73C, ^3E, and 73F are 

 copied from Walcott [1908d, PI. X, flgs. 9, 9a-c, respectively]. 



shells by a broad median sinus that disappears on the umbo ; area short and divided midway by a 

 relatively large, open, triangular delthyrium. Dorsal valve convex with an elevated, rela- 

 tively narrow median fold that does not extend back to the beak; area short and divided by 

 a strong, open, triangular delthyrium. 



Surface marked by concentric striae and a few strong lines of growth. The largest shell 

 is represented by a dorsal A^alve that has a length of 12 mm.; width, 14 mm. A ventral valve 

 7 mm. in length has a width of 1 1 mm. 



Casts of the ventral valve show a spondylium supported on a septum that extended from 

 the beak about one-third the distance to the front margin. The spondylium of the dorsal 

 valve rests directly on the interior of the shell without trace of a supporting median septum. 

 62667°— VOL 51, pt 1—12 51 



