NO. 1152. CAMBRIAN BRA CHI OPODA— WALCO TT. 387 



of tlie shell, exteDtling to within a short distance of the frontal margin; 

 in others it is limited to the posterior half of the shell. The same fea- 

 tures occur in the dorsal valve. There are no traces of a median septum 

 in the ventral valve; in the dorsal valve it is shown in the cast as a 

 very narrow depression between and a little forward of the central 

 muscle scars. 



In the ventral valve the anterior lateral muscle scars are distinctly 

 shown; also the trapezoidal area, in which the central, middle, and 

 outside lateral scars occur. In the dorsal valve large central and small 

 anterior lateral scars are clearly defined; also the transmedian scars. 

 Of the markings left on the interior of the shell by the vascular system, 

 the trunk sinuses are usually strongly defined in the smaller shells, 

 extending nearly to the anterior margin, and in the larger shells about 

 three-fourths of the way over the area to the frontal margin. 



Owing to the condition of the casts of the interior, the parietal scar 

 is usually not well defined; in the ventral valve it appears to extend 

 from where it arches forward at the center almost directly outward to 

 the trunk sinuses, where it curves backward across the sinus and 

 outside of the anterior lateral muscle scars; it occurs in the dorsal 

 valve over the median line in front, is outward and backward around 

 the side of the large central muscle scar, whe e it curves outward 

 across the trunk sinuses. 



Observations. — Attention has been (jailed to the relatively large size 

 of the visceral cavity (Splanchnoccele) in both valves of the smaller 

 shells. The. range of variation in this respect is so great, that it might 

 be accepted as indicating a distinct species if there were not shells 

 intermediate in size in which the splanchnocade was also intermediate 

 in proiDortional size. Another marked character in the specimens is 

 the very strong impressions in the cast of the trunk sinuses and muscle 

 scars and visceral markings. This species is somewhat more rounded 

 in outline than 0. mwra and 0. matinalis, and it is very distinctly 

 marked by the muscle scars of the dorsal valve. 



Fonnation and locality. — Middle Cambrian, St. Croix sandstone, 

 Hudson, Wisconsin, 



Type — No. 27299, U.S.N.M. 



OBOLUS RHEA, new species. 



G-eneral form elongate ovate, with the ventral valve subacuminate, 

 and the dorsal valve elongate ovate in outline. Outer surface unknown, 

 as all of the shells referred to this species are more or less exfoliated. 

 The surface of the inner layers shows numerous, rather broad, radiating 

 strite, and concentric lines of growth. The shell api)ears to have 

 been formed of a thin outer layer and several inner layers or lamellie 

 arranged in the same manner as in 0. matinalis. A ventral valve 8 

 mm. in length has a width of G mm.; a shorter broader valve is (\.^ 

 mm. long and 5.5 mm. wide. The two dorsal valves referred to this 



