CAMBRIAN BRACHIOPODA — WALCOTT 55 



in a distance of two millimeters. Fine papillae are thickly scattered 

 over the surface. They have a tendency to follow concentric lines 

 of growth on some portions of the shell, and on others they appear 

 on low, narrow, serpentine ridges, as in Mickwitsia monilifera (Lin- 

 narsson) [1869, p. 344]. A few large pimctse are scattered here 

 and there over the surface. Inner surfaces and layers of shell 

 unknown. 



This beautiful shell differs in the details of its surface from M. 

 monilifera; it is also less convex and the apex is nearer the posterior 

 margin. 



Formation and Locality. — Lower Cambrian : Eophyton sand- 

 stone, at Lugnas, Vestergotland, Sweden. 



Genus MICROMITRA Meek [1873, p. 479] 

 MICROMITRA HAYDENI, new species 



Plate 7, Figures 3 and 2i<^ 



Ventral valve subconical, with a minute beak arching slightly over 

 a strong, arched pseudodeltidium, which is about one-half as long as 

 the height of the valve. Cardinal slope rounded ; a slight angle is 

 indicated by a line where the concentric surface strise bend inward 

 toward the pseudodeltidium across the narrow area; a sharp angle 

 is formed where the convex pseudodeltidium rises abruptly from the 

 area. 



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 as far as now known. 



Surface marked by fine, concentric, slightly undulating, thread- 

 like strise and a varying number of irregular, more or less inter- 

 rupted, narrow, depressed, rounded, radiating- ridges ; these ridges 

 are usually most numerous at the central portions of the valves. 

 The concentric striae 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 mm. in length by 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 lamellae. 



Observations. — Micromitra hoydeni differs from the nearest re- 

 lated species, M. scnlptilis (Meek) [1873, p. 479], in having a strong, 



