806 CAMBRIAN BRACHIOPODA. 



The ventral valve has a broad, strong sinus that depresses the front of the valve; area 

 well defined, with a large, open delthyrium. Casts of the interior of the ventral valve show 

 a well-marked spondylium and very strong main vascular sinuses, figure 2b; both spondjdium 

 and sinuses recall those of BiUingsella plicateUa (PI. LXXXVI, figs. 3g, 3j). Sections of the 

 ventral valve, cut across the umbo, show the spondylium attached to the bottom of the valve. 

 The flabelliform diductor and adductor muscle scars are clearly shown outside of the vascular 

 sinuses. 



Dorsal valve with an elevated median fold that gives a strong convexity to the valve; 

 area low, with a strong, open delthyrium; the interior of the valve shows a shallow spondylium 

 attached to the bottom of the valve posteriorly, and probably supported by a low median septum 

 toward the front (figs. 2i, 2j, section fig. 2m) and well-defined anterior and posterior adductor 

 muscle scars; narrow main vascular sinuses occur (fig. 2h), and slender vascular lines radiate 

 forward from the muscle scars (fig. 2f). 



Observations. — This species differs from all allied forms in its variation in outline, con- 

 vexity, ribs, and internal markings. Some shells approach closely to those of Huenella texana 

 (Walcott) (compare figs. 1 and 2 and figs, lb and 2a, PI. CIII), but each species has a majority 

 of shells that are quite unlike those -of the other. Another point of resemblance is the form of 

 the spondylium as shown by cross sections. A variety of H. texana from Cold Creek Canyon, 

 Texas, has some shells that approach those of H. abnormis in the extravagant development of 

 the fold on the dorsal valve and sinus on the ventral valve, but in other respects they differ from 

 it. The form of spondylium and plications or ribs suggests the genus Parastrophia Hall. 



This form was an abnormal representative of the genus SyntropJiia, to which it was first 

 referred, hence the specific name. 



Formation and locality. — Upper Cambrian: (153) Limestone in ravine on west side of Dry Creek, near the 

 mouth of Pass Creek, about 5 miles (8 km.) north of Hillsdale; and (157 and 158) limestone north of East Gallatin 

 River, near Hillsdale; both on the Threeforks quadrangle (U. S. Geol. Survey), Gallatin County, Montana. 



(302p) Middle limestone of the Deadwood formation, near the summit of the Owl Creek Mountains, 18 miles (29 

 km.) southwest of Thermopolis, Fremont County, Wyoming. 



Middle Cambrian: (4j) Limestone at the head of Deep Creek, Yellowstone National Park, Canyon quadrangle {U. S. 

 Geol. Survey), Wyoming. 



^ Huenella billingsi (Walcott). 



Plate CII, figures 5, 5a-c. 



Syntrophia billingsi Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 291-292. (Described and discussed as below 

 as a new species.) 



General form transversely subquadrate; rounded; biconvex, with the dorsal valve elevated 

 on the median fold. Surface marked by a few imperfectly developed ribs on some shells, while 

 others have only concentric strife and lines of growth. Ventral valve with a relatively shallow 

 median sinus, in which obscure, narrow ribs sometimes occur; none of the specimens in the col- 

 lection show the area, but from the profile of the valve it must have been of moderate height, 

 with the rather sharp apex curvmg slightly over it; a single poor cast of the interior shows 

 the outline of a spondylium much like that of S. primordialis; the evidence of the presence of 

 a median septvuii is a dark line in front of the end of the cast of the spondylium, which indicates 

 that the shell substance extends down into the limestone and that it is the median septum. 

 Dorsal valve with a small umbo and apex that extend forward into a prominent median fold 

 that may be plicated (PI. CII, fig. 5c) or smooth; one partly exfoliated shell shows three plica- 

 tions on the fold and three or four on each lateral slope of the valve. 



Observations. — This species belongs to the plicate group of the Syntropliiidse represented 

 by Huenella texana (Walcott) and H. abnormis (Walcott). It differs from H. texana in being 

 less convex and in having a shallower ventral sinus, less prominent dorsal fold, and more obscure 

 plications. 



The specific name was given in honor of Mr. E. Billings. 



Formation and locality. — Middle ? Cambrian: (28a) "St. Albans formation," in limestone lentil about 1 mile 

 (1.6 km.) east of Parker's quarry, west of Georgia, Franklin County, Vermont. 



