58 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 139 



the two genera, which look similar, actually are developed differently. 

 In the Paleozoic genus the median septum is strong and high and 

 supports a V-shaped chamber having strong walls. This is in turn 

 covered by a flat plate connecting the crural bases. This plate is ap- 

 parently built, at least in part, posteriorly because it does not gen- 

 erally close the chamber but leaves a small round cavity at the apical 

 end. This chamber of Camarotoechia is more like the structure in 

 Septaliphoria. The chamber of this Jurassic shell is, however, also 

 different from that of Camarotoechia, although the two look alike. 

 The chamber of Compsothyris is a much more delicate affair and, it 

 seems to me, not related to any Paleozoic form. 



GRAMMETARIA Cooper, new genus 



(Gr. gramme, line) 



Plates 4, C, 9, A 



Outline elongate triangular with the maximum width at the ante- 

 rior; valves subequal in depth, the pedicle valve having a slightly 

 greater depth ; anterior commissure rectimarginate ; surface marked by 

 fine costellae. Beak small, bluntly pointed, suberect ; foramen incom- 

 plete, rounded, hypothyrid ; deltidial plates auriculate, conjunct. 



Pedicle valve interior with small corrugated teeth, supported by 

 strong vertical dental plates. Muscle field small and subcircular ; 

 diductor scars small; adjustor scars large, posterolaterally placed. 



Brachial valve interior with corrugated sockets bounded by strong, 

 thick socket ridges ; crura short, of spinulif er type, triangular in sec- 

 tion but laterally flattened distally, attached to the socket ridges by 

 very narrow, inconspicuous outer hinge plates ; median ridge thick, 

 not quite reaching the apex in the adult, but in the young forming 

 a low, wide V-shaped chamber with the crural bases ; V-shaped 

 chamber covered by shell substance in the adult. Adductor field 

 elongate triangular, the anterior and posterior scars on each side of 

 the median ridge tear shaped in outline; posterior set of adductors 

 located outside the anterior set. 



Type species. — Hemithyris bartsclii Dall, Proc. U. S. Nat. Mus., 

 vol. 57, p. 289, 1920. 



Comparison. — This genus is most suggestive of Frieleia and 

 Compsothyris among described genera, but it has important differ- 

 ences from both of them. In the first place, Frieleia has a strong 

 tendency to ligation while Compsothyris is broadly uniplicate. Gram- 

 metaria, on the other hand, is rectimarginate. The deltidial plates of 

 the pedicle valve of Grammetaria are elaborately auriculate but such 



