338 



CAMBRIAN BRACHIOPODA. 



Dorsal valve moderately convex, most elevated at the small umbo just in advance of the 

 marginal minute beak ; area very low and narrow, and without trace of pseudodeltidium so far 

 as now known. 



Surface marked by fine, concentric, slightly undulating, threadlike strise and a varying 

 number of ii-regular, more or less interrupted, narrow, depressed, rounded radiating ridges; 

 these ridges are usually most numerous at the central portions of the valves. The concentric 

 strias extend across the narrow area and arch over the pseudodeltidium, where they are finer 

 and crowded together so that all the strise between the apex and the front margin are com- 

 pressed in about one-half the distance on 

 the pseudodeltidium. The adult ventral 

 valve is about 4.5 mra. in length hj 5 

 mm. in width and 2.5 mm. in height, with 

 a pseudodeltidium 1.3 mm. in length. A 

 dorsal valve 2 mm. in length has a height 

 of about 0.5 mm. at the umbo. The shell 

 is rather thick for a species of this size, 

 and it is built up of several thin layers 

 or lamellas. 



Ohservations. — Micromitra Tiaijdeni 

 differs from the nearest related species, 

 M. sculptilis (Meek), in having a strong 

 convex pseudodeltidium, less elevation 

 of the ventral valve, and a thicker shell. 

 Alicromitra Jiaydeni occurs near the base 

 of the Middle Cambrian and M. sculjjtilis 

 about 2,000 feet (610 m.) higher in the 

 section of the Cambrian rocks of Utah and 

 southern Idaho. 



The specific name is given in honor of Dr. F. V. Hayden, geologist and explorer, under 

 whose charge the geology of this region was first studied. 



Formation and locality. — Middle Cambrian: (54s) Dark blue-gray Langston limestone [Walcott, 1908f, p. 198], 

 just above the Cambrian quartzitic sandstones, north side of Twomile Canyon, near its mouth, 2 miles (3.2 km.) south- 

 east of Malade, Oneida County, Idaho. 



FiGUKE 20.— Micromitra hayieni Walcott. A, Top view of ventral valve (U. S. 



Nat. Mus. Cat. No. 51437a). B, B', Side and baclc views of a ventral 



valve (U. S. Nat. Mus. Cat. No. 51437b). C, E.xterior of dorsal valve (U. S. 



Nat. Mus. Cat. No. 51437c). D, Dorsal valve (U. S. Nat. Mus. Cat. No. 



51437dV 

 The specimens represented are from Locality 54s, Middle Cambrian lime- 

 stones, near Malade, Idaho. Figures 20A and 20B' are copied from Walcott 

 [1908d, PI. VII, figs. 3 and 3a]. Figure 20A represents the type specimen. 



\y 



Micromitra nisus (Walcott). 

 Plate II, figure 10. 



Iphidella nisus Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 305. (Described and discussed as below as a new 

 species.) 



Ventral valve rather low, with the apex overhanging the posterior margin. Surface 

 marked by very fine lines of growth with still finer bands of strise between them. About 10 

 very fine radiating ridges extend from near the apex to the front and lateral margins. 



This little shell is represented by a single specimen of the ventral valve. The associated 

 fauna includes fragments of Olenellus. 



I was at first inclined to refer this shell to Micromitra sculptilis (Meek) or M. pealei (Wal- 

 cott), but the overhangmg apex and strongly marked surface and the fact that there is an 

 interval of 2,000 miles between the localities of the species led me to separate them. Strati- 

 graphically the two latter species occur in the Middle Cambrian and M. nisus in the Lower 

 Cambrian. I do not think the two forms should be included in one species. 



Formation and locality. — Lower Cambrian : (2r) In a limestone bowlder in a conglomerate, in a railroad cut 2 

 miles (3.2 km.) west of Bio railway station, Rimouski County, Quebec, Canada. 



