62 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I39 



Plicirhynchia. The socket ridges are thick and curved ; the crura are 

 attached to them by small, triangular outer hinge plates. The crura 

 are laterally compressed blades, not concave in section and their distal 

 end is serrated. Prominent inner hinge plates are formed at the apical 

 end of the shell which attach to the floor of the valve. With the 

 septum they form a poorly defined chamber but in old specimens the 

 inner plates become thickened at the apex and fuse to form a thick 

 callus. In such cases the extensions of the inner plates to the valve 

 floor are obscured. 



The adductor field is small and elongated. The anterior pair of scars 

 is the larger and the posterior pair more elongated, at least in the old 

 shells. The field is divided medially by a short, thick median ridge 

 which extends no farther than the anterior end of the adductor field. 

 At the anterior end of the ridge a small scar appears in old specimens. 

 The genital areas are small like those of the pedicle valve. The vascula 

 media originate at the inside ends of the anterior adductors and sur- 

 round the small scar mentioned above. These pallial trunks divide 

 near mid valve in a manner similar to that of the pedicle valve. 



Comparison of Hispanirhynchia fcraneana with middle-aged speci- 

 mens of H. cornea show slight differences but they do not appear to be 

 great enough to exclude the species from Hispanirhynchia. The outer 

 hinge plates of the brachial valve appear slightly wider than those of 

 the Atlantic shells and the inner hinge plates are not so well developed, 

 but they are there. 



Rhynchonella sicnla Seguenza, here made the type of the genus 

 Sphenarina, was early identified with H. cornea, but examination of 

 the interior of the Sicilian shell makes it clear that the two have little 

 in common but shape and ornament. Sphenarina has no median 

 septum and its beak characters are different from those of Hispani- 

 rhynchia. 



SPHENARINA Cooper, new genus 



(Gr. sphenos, wedge) 



Plates 5, B, 8, A 



Shell triangular in outline, with the greatest width in the anterior 

 third; subequivalve, the pedicle valve having a slightly greater depth 

 than the brachial valve ; anterior commissure rectimarginate ; surface 

 marked by minute radial lines. Beak long, suberect ; foramen small, 

 circular, hypothyrid; deltidial plates conjunct, elaborately auriculate. 



Pedicle valve interior with short pedicle collar and well-developed 

 dental plates with wide umbonal cavities. 



