BRACHIOPODA. 71 



The latter author also describes and figures* a brachial 

 valve of 0. gemma from Bic Harbor, below Quebec, in which 

 the cardinal area is quite narrow and bears a deep pedicle- 

 groove. It is possible that this feature may be subject to so 

 great variation within the limits of the same species as in- 

 dicated by Mr. Walcott's figure and our own; but this point 

 renuirps verification ^"*-^''- Brachial valve of 



rcquiieb VCIlUCailon. o. gemma. After Walcott. 



In the pedicle-valve the groove on the area is followed by a deep pedicle 

 muscular impression, which is flanked on either side by narrow, elongate cardi- 

 nals. The lateral scars begin near the pedicle-pit or scar and curve gently 

 outward, and again inward near their distal extremities, extending two-thirds, 

 sometimes three-fourths the length of the shell. They are not always a simple 

 groove, but in 0. crassa each margin of the impression is more deeply sunken 

 than the rest, thus leaving a low ridge between them. In the post-median 

 region, the central scars are usually ill-defined but evidently compound, and 

 in 0. crassa are seen to consist of three subcircular scars, two outer and one 

 inner ; these are bordered on the anterior side by a series of thread-like ridges 

 and furrows. In 0. gemma the entire central scar is more deeply impressed, 

 and partakes of the tripartite character, the median portion being the most 

 pi-ominently developed. 



In the brachial valve, the laterals take their origin in the median region 

 of the shell, adjacent to, or in the central impressions, and curve outward. 

 An excellently preserved interior of this valve of 0. crassa shows a subrhom- 

 boidal central scar with straight sides and with the apex pointed posteriorly ; 

 the margins are more depressed than the center, but the impression is not 

 divided by a ridge into two scars, as represented in Mr. Ford's figure.f The 

 laterals branching from this scar are very broad and slightly curved. In other 

 species the central impression is usually indefinite in its details, as in the pedi- 

 cle-valve, but shows a subdivision into outside and median scars. 



The cardinals in 0. crassa are well defined and, at their anterior extremities, 



* Bulletin No. 30, U. S. Geological Survey, p. 117, pi. x, fig-. Id. 

 t Bulletin No. 30, U. S. Geological Survey, pi. x, fig. 1 d. 



