56 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I39 



Dall ( = Terebratula nitens Conrad) has interior characters strongly 

 suggesting Frieleia, especially the strong median septum in the 

 brachial valve. The species is fairly strongly uniplicate, however, 

 which is not in accordance with the current definition of Frieleia. 

 All the specimens of H. astoriana available for study, including the 

 type specimen, are badly exfoliated. The exterior is therefore not yet 

 wholly known. The exfoliated shells have fairly strong radial costel- 

 lae, but these may be only a feature of the exfoliated shell. A cross 

 section of the beak of the brachial valve reveals a small triangular 

 chamber. No modern specimens of Frieleia are uniplicate. The 

 specimens of H. astoriana are here referred to Frieleia with a query. 

 They are nearer that genus than they are to Hemithyris. Ultimately 

 it may be necessary to erect a new genus for uniplicate Frieleia if 

 specimens good enough for detailed description are brought to light. 



Genus COMPSOTHYRIS Jackson, 1910 



Plate 16 



Compsothyris Jackson, British Antarctic ("Terra Nova") Exped., 1910, Nat. 

 Hist. Rep., Zool., vol. 2, No. 8, p. 188, 1918; Thomson, New Zealand Board 

 Sci. Art, Manual 7, p. 161, 1927. 



Rounded triangular in outline with the greatest width at about the 

 middle ; valves subequal in depth, the pedicle valve having a greater 

 depth than the brachial valve ; anterior commissure broadly and gently 

 uniplicate, the brachial fold inconspicuous. Surface marked by fine 

 radial costellae. Beak of pedicle valve nearly straight to suberect, 

 bluntly pointed; foramen incomplete, of moderate size, elongate 

 elliptical, hypothyrid (permesothyrid according to Jackson, 1918) ; 

 deltidial plates disjunct. 



Pedicle valve interior with small teeth supported by strong dental 

 plates ; muscle field located well anterior to the delthyrial cavity, small ; 

 diductor scars small, surrounding the adductor pair. Pallial marks 

 not impressed. 



Brachial valve interior with narrow corrugated sockets bounded by 

 strongly overhanging socket ridges; crura of spinulifer type, short, 

 attached to the socket ridges by narrow hinge plates. Inner hinge 

 plates incipiently developed. Median ridge or myophragm slender, 

 moderately elevated and reaching the apex where it is divided and 

 supports the proximal ends of the crural bases. Adductors closely 

 crowded against the myophragm, the right and left pairs being tear 

 shaped in outline. 



Type species (by original designation). — Rhynchonella racovitzae 



