NO. 1152. CAMBRIAN BRACHIOPODA—WALCOTT. 417 



marked by concentric lines and stria' of j;rowth ; when the outer layer 

 is exfoliated radiating striic cross the lainelljc; a fragiiieut of the inner 

 surface of the shell indicates that there were scattered pits or punctai 

 and fine radiatinf>- stria*; the shell was relatively thick, and formed of 

 a thin outer layer and several inner layers or lamella-, the lamella- of 

 the interior portion of the shell being arranged "in layers slightly 

 obli([ue to the outer surface of the shell. 



C'asts of tlie interior of the ventral valve show a well defined area 

 mark( d by strong flexure lines that occur midway between the lateral 

 margins and the narrow, well defined pedicle groove; stria; of growth 

 cross tiie area i)aral!el with its base. The area of the dorsal valve is 

 relatively short and does not extend very far out on the cardinal slopes. 

 The interior markings of the ventral and dorsal valves show imper- 

 fectly the main vascular sinuses and visceral area. 



Ohservadon.s. — This species ()c(;urs at the same relative stratigraphic 

 horizon as 0. {L.) ino, and is about the same size. It differs, however, 

 in its more circular form, which is persistent in a large number of 

 shells. In forn) it more nearly resembles ().{L.) rofwuUttns of the 

 Ui)per Cambrian, it differs from that in being uniformly larger and 

 having a thicker, stronger shell. It is also not piobable that a Middle 

 Cambrian species would continue to exist until the close of Cambrian 

 time. 



Formation I'nd locality. — Middle Cambrian, biown sandstone beneath 

 the alternating layers of sandstone and limestone northwest end of 

 Tacksaddle Mountain, Llano County, Texas. 



Tijpc.—^o. 27341, U.S.N.M. 



OBOLUS (LINGULELLA) TARPA, new species. 



General form elongate ovate, with the ventral valve subacuminate, 

 and the dorsal valve ovate in outline. There is considerable range of 

 variation in the outline of both valves, owing in ])art possibly to dis- 

 tortion. The convexity of the two valves is fairly strong, as far as 

 can be determined from the s(nnewhat <;om[)ressed condition of the 

 shells in the shale and calcareous sandy shales. The largest ventral 

 valve in the collection has a length of 14 mm. The average length of 

 the ventral valve is from 10 to 11 mm. One 11.5 in length has a width 

 of 8 mm. 



None of the specimens of the collection show the outer surface, and 

 only traces of (M)ncentric and radiating lines have been observed on 

 the inner surface. The shell appears to be moderately thick and 

 formed of numerous lamella- that were obliijue to the outer layer in 

 the anterior portions of the shell, in this respect resembling the shell 

 of 0. (/>.) acutanfiuluH. 



Casts of the interior of the ventral valve have a moderately long area 

 divided midway by the cast of a strong pedicle furrow, and marked 

 about midway between the pedicle furrow and the lateral margin by a 

 Proc. N. M. vol. xxi 27 



