NO. 1152. CAMBRIAN BRACHIOPODA—WALCOTT 389 



outer surface of the inner layer is marked by very fine radiating striie 

 and numerous lines of growth. The inner surface of the shell is nearly 

 smooth, Judging from a partial cast in the limestone. The shell is rela- 

 tively thin and formed of a very thin outer layer and one or more thin 

 inner layers or lamcllic, which thicken the shell from the umbonal 

 region and toward the front and sides. 



A (;ast of the interior of a dorsal valve that is referred to this species 

 shows tiiat a strong median ridge was present; also a median septum 

 and a strong main vascular sinus. The area is short and marked by 

 rather prominent flexure lines, as indicated by the flexures in the 

 transverse lines of growth. 



Observations. — This species might be taken for the young of 0. mati- 

 nal'iH or O. mwra,, were it not for the great difference in the thickness 

 of the shell. It occurs at a slightly higher horizon at the base of the 

 Pogonip limestone. 



Formation and locality. — Lower Ordovician, lower portion of Pogonip 

 group, northeast of Adams Hill, Eureka district, Nevada. 



Type.—l^o. 27302, U.S.N.M. 



OBOLUS LOPERI, new species. 



General form subsemicircular, with the ventral valve subacuminate, 

 and the dorsal valve broad ovate to circular in outline. Home of the 

 shells are more elongate than in what is considered to be the typical 

 form. This type of variation is also observed in 0. matinalis and other 

 species of the genus. Valves moderately convex as they occur in the 

 sandstone. Surface of the shell mai»ked by concentric lines and fine 

 striii' of growth, and very narrow radiating undulations that are more 

 or less interrupted by the concentric; lines of growth. When the outer 

 layer of the shell is exfoliated the inner layers are seen to be marked 

 by numerous fine, rounded, radiating stria', in addition to the concen- 

 tric lines of growth, exceedingly fine, irregular, interrui)ted stria- that 

 give it in places a pitted ai)pearance, while in a different light it appears 

 to be granulated, a feature of the surface that seems to be present on 

 the surface of all of the inner lamella',; sometimes the impression given 

 is that the shell is minutely i)unctate. 



The markings of the interior, so far as known, are rounded radiating 

 stria?. The shell is rather thick and built up of a thin outer layer 

 and several inner layers or lamella' that in the anterior i)ortions of the 

 shell are rounded obliquely to the outer surface. 



The largest shell in the collection is a somewhat imperfect dorsal 

 valve 8 mm, in length. A smaller valve, 6 mm. in length, has the 

 same width. A ventral valve, mm. in length, has the portion about 

 the beak broken away and is a little longer than wide. 



The only traces of the interior of the valves is a partial cast of the 

 dorsal valve. This shows that the interior lateral muscle scars were 



