404 CAMBRIAN BRACHIOPODA. 



interior of tlie valves show very strong concentric undulations and lines of growth, although 

 in some specimens these characters are scarcely perceptible. The fragments of the shell pre- 

 served show that it was formed of a thin outer layer, several inner layers or lamellae of varying 

 thickness, and numerous lamellae over the anterior and lateral portions of the shell that are 

 shghtly obhque to the outer surface. A somewhat rounded ventral valve has a length of 

 9 mm., width 9 mm. ; a dorsal valve 9 mm. long has a width of 8 mm. ; a more elongate ventral 

 valve is 9 mm. in length and 8 mm. in width; an associated dorsal valve 7.5 mm. in length 

 has a width of 7 mm. 



Casts of the interior of the ventral valve show an area of medium length, divided midway 

 by the cast of a strong, rather broad pedicle furrow, and again a short distance each side of the 

 pedicle furrow by a narrow, sharp, flexure line; stri^ of growth cross the area of the pedicle 

 furrow parallel with the front margin. There is slight evidence in one of the casts that the area 

 formed a shelf between the pedicle groove and the lateral margin. The area of the dorsal valve 

 is of medium length and fairly well extended out onto the cardinal slopes (PI. X, fig. Id). The 

 cast of the visceral cavity of the ventral valve is well shown by several specimens (PI. X, figs. 

 la, lb, and Ic). It resembles that of Oholus matinalis (Hall) and 0. apollinis quenstedti (Mick- 

 witz) in the extension of the anterior margins almost directly outward from the center toward 

 the impression of the main vascular sinuses ; one of the peculiarities of the species is the great 

 development of the area within the parietal scar (splanchnocoele) ; in some examples (PI. X, 

 figs, la and lb) it occupies all the central portions of the shell, extending to within a short 

 distance of the frontal margin. In others it is limited to the posterior half of the shell; the 

 same features occur in the dorsal valve (PI. X, figs. Id, le, and If). 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 trape- 

 zoidal area, in which the central, middle lateral, and outside lateral scars occur. In the dorsal 

 valve (PI. X, figs. Id and Ig) large central (h) and small anterior lateral (j) scars are clearly 

 defined, also the transmedian (i) 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, extend- 

 ing 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; in the dorsal valve it extends from over the median 

 line in front outward and backward around the side of the large central muscle scar, where it 

 curves outward across the trunk sinuses. 



Observations. — Attention has been called to the relatively large size of the visceral cavity 

 (splanchnocoele) 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 splanchnocoele is also intermediate in proportional 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 outhne than 0. msera (Hall and Whitfield) and 0. matinalis (Hall), and it is very distinctly 

 marked by the muscle scars of the dorsal valve. 



The specific name was given in honor of Dr. A. Mickwitz. 

 Formation and locality.— Upper Cambrian: (79, 79', and 791)) « "St. Croix sandstone, "near Hudson; and (328j) 

 "St. Croix sandstone' ' on Willow Kiver, near Hudson; both in St. Croix County, AVisconsin. 



Obolxjs minimus Walcott. 



Plate XI, figures 8, 8a. 



Obolus minimus Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 325-326. (Described as below as a new species.) 



This is a small shell of the general form of Oholus sTiansiensis Walcott. The ventral valve 

 is obtusely acuminate and the dorsal nearly circular; valves gently convex. Surface marked by 



a 79 is the type locality. 



