20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I39 



orly to disappear before reaching the apex in young shells. In old 

 specimens a low extension of the septum extends to the apex where 

 it unites with a thickening from the cardinal process. The septum 

 thus makes a narrow wedge extending ventrally almost to the inner 

 wall of the pedicle valve. 



I have not observed the radial striae reported by Dall (1920, p. 293) 

 in young shells. 



The fossil species assigned doubtfully to Cryptopora may be the 

 young of other species. The gaping foramen and rudimentary deltidial 

 plates are suggestive of young rhynchonelloids. Meznerics (1943, 

 p. 23) points out that Sacco believed H. parvillima to be a juvenile of 

 H. de buchii—Streptaria buchi. 



I have examined a specimen of Mannia nysti Davidson from the 

 Miocene of Belgium. As explained in the discussion under Mannia, 

 this specimen has the features of Cryptopora but does not conform 

 completely with the description given by Davidson. The description 

 of this genus is evidently inaccurate and the two genera are exact 

 synonyms (see discussion under Mannia). 



CRYPTOPORA RECTIMARGINATA Cooper, new species 



Plates i, B, 2, A 



Atretia gnomon Dall (not Jeffreys), Proc. U. S. Nat. Mus., vol. 57, p. 293, 

 1920 (U.S.N.M. Cat. Nos. 83131, 274138, 274139, 94367, 336894). 



Shell small, translucent to white, subtriangular in outline, with the 

 greatest width anterior to the middle ; sides gently rounded ; anterior 

 margin strongly rounded; valves subequal in depth; anterior com- 

 missure rectimarginate ; surface smooth. 



Pedicle valve slightly deeper than the brachial valve; lateral pro- 

 file gently convex, most convex in the posterior third and flattened 

 in the anterior third; anterior profile broadly convex, slightly more 

 convex than the brachial valve in this profile. Beak pointed, forming 

 an angle of about 85 , suberect; deltidial plates erect, thickened along 

 their distal margin, commonly extravagantly auriculate, the auricula- 

 tions directed laterally. 



Pedicle valve interior with thick apical plate well elevated above the 

 valve floor ; teeth small, wide ; dental plates stout, slightly divergent an- 

 teriorly, approximately vertical to the valve floor. Muscle field anterior 

 to delthyrial cavity. 



Brachial valve with gently convex lateral profile, the maximum con- 

 vexity located just anterior to the umbo and posterior to the middle; 



