10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



GEMMARCULA MENARDI (Lamarck) 

 Plate 2B, figures 29-34 



Figures of the interior and exterior of this fine species are intro- 

 duced for comparison with G. arizonensis. Note elaborate develop- 

 ment of the cardinal process in this European species. 



Horizon and locality. — Cretaceous (Green Sand), LeMans, Sarthe, 

 France. 



PSILOTHYRIS Cooper, new genus 



Generally small to medium size, oval to subpentagonal in outline; 

 anterior commissure rectimarginate to uniplicate ; valves unequally 

 convex, the pedicle having the greater depth and convexity. 



Pedicle valve with strong beak ridges ; beak erect ; foramen round, 

 small to large, submesothyrid to mesothyrid ; deltidial plates disjunct 

 to conjunct, often worn away by pedicle pressure. Interior with stout, 

 thick teeth, buttressed by stout dental plates ; no pedicle collar ; muscle 

 scars not discernible in available material. 



Cardinalia small ; hinge plate undivided, short, often upturned on its 

 anterior edge; inner socket plate concave, stout; crura short, stout; 

 crural process long and slender ; loop simple, long dalliniform, forming 

 a broad ribbon and having a broad transverse band in the adult; 

 median septum of the adult short, slender, and may or may not but- 

 tress the hinge plate, and reduced to a faint myophragm at the valve 

 middle. Young stages with loop metamorphosis like that of the Dal- 

 linidae. 



Type species. — Psilothyris occidentalis Cooper, new species. 



Discussion. — This species is characterized by its smooth exterior, 

 simple uniplicate folding, short inner hinge plate and long dalliniform 

 loop. Details of the formation of the hinge plate and the development 

 of the loop are discussed under the specific description. The combina- 

 tion of characters exhibited by Psilothyris is different from any de- 

 scribed Cretaceous dallinoid and is also unlike any described smooth 

 Jurassic zeilleroid and dallinoid. 



Of smooth Jurassic zeilleroids that resemble Psilothyris more or less 

 closely, Zeilleria, Microthyridina, Rugitela, and OrnithcUa are pro- 

 vided with a strong median septum in the brachial valve and the fold- 

 ing of all of them is different from that of Psilothyris. Aulacothyris 

 and Antiptychina are differently folded, these two genera having 

 a strongly sulcate brachial valve. The dallinoids Plesiothyris and 

 Obovothyris have long septa and different folding. Epicyrta has a 

 carinate brachial valve and is thus quite different externally. 



