K0.1152. CAMBRIAN liRACniOPODA—WALCOTT. 403 



and inoro so tlian those of 0. (L.) aurora, the result being- a surface 

 character iiitcniiediato between that of these two species. When the 

 outer layer is exfoliated, the surfa(5<'- of the inner hiyer is marked by 

 numerous tine, radiating- stria* and concentric lines of growth. The 

 cast of the inner surface of the shell shows rather numerous papilhe 

 that lill the i)its or punctje in the shell. The shell is strong and formed 

 of a thin outer layer, and several inner layers or lamella" that are 

 arranged very much as in (). {L.) acutaiu/ulm. 



As shown by casts of the interior, the cardinal area of the ventral 

 valve is rather long and well extended out on tlie <;ar(linal slopes. It 

 is divided at the center by a cast of a stiong, rather deep pedicle fur- 

 row, and about three-iifths of the distance between the pedicle furrow 

 and the latcu-al nuirgin by a sharp, narrow Mexure line. The stria; of 

 growth cross the arc^a parallel to its base. Only a few traces of them 

 are preserved in the pedicle furrow. The area formed a thin shelf 

 between the pe(li(;le groove and the lateral margins, the undercut 

 extending far back under the area as in O. (L.) avHtaiujaluii. This is 

 shown in the cast by a thin projection of the embedding rock over the 

 area. The area of the dorsal valve is low(!r and less prominent. It 

 arclies forward at the median line and exteiuls well out on the cardinal 

 sloi)es. 



The cast of the visceral cavity of the ventral valve includes the 

 heart-shaped ])it and a slight trace of the trapezoidal area, in which 

 the central, middle, and oulside lateral nuisole scais occur. There are 

 no traces of a median septum in the ventral valve, and it is only 

 slightly indicated in one specimen of the dorsal valve. This is owing, 

 however, more to the condition of i)reservation of the specimen tiian 

 to the character of the septum. No muscle scars are clearly delined 

 in either valve. Of the visceral system the main or trunk sinuses are 

 fairly well shown in the ventral valve, but less so i'or the doisal valve. 



Ohservatioiifi. — This form has the shape of 0. {Ij.) acu(an(jnl((s, hut 

 differs radically in the arrangement of the markings on the interior of 

 the shell. This is especially true of the dorsal valve. In 0. {L.) eiif/Ii/- 

 plian the traces remaining on tiie casts iiulicate a very close resem- 

 blance to (). (L.) chuaremis. The thickness of the shell also allies it 

 with OboliiH rather than the subgenus Linf/iilella. Attention has been 

 called to the character of the surface, which is intermediate between 

 that of <). (L.) ella aiul (). (/>.) aurora. 



This species is associated with (). [L.) lineolatus in the upper beds of 

 the Ton to sandstone. It dilfers from that species in its surface charac- 

 ters, thickness of sliell, and usually in outline. It is also usually a 

 larger species, although a few examples of (). {L.) Hucolatnti ap[)roach 

 it in size. 



Formal ion and, locality. — Middle (Jambrian, Tonto sandstone at the 

 head of Lava and Nuukoweai) valleys, (Irand Canyon of the Colorado, 

 Arizona. 



7V/>6'.— No. 27;51(;, U.S.N.M. 



