LAMELLIBRANCHIATA. 157 



Glyptodesma occidentale. 



PLATE XV, FIG. 12; AND I'LATK LXXXVI, FIG. 9. 



Glyptodesiim occidentale. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 15, tig. 12. 



Jan., 1883. 



Shell large, broadly ovate ; body nearly erect ; height and length about equal ; 

 margins regularly curved. 



Left valve very convex, gibbous on the umbo. Right valve unknown. 



Hinge-line straight, equalling or greater than the length of the shell. 



Beak anterior to the middle of the shell, directed slightly forward, acute 

 and prominent. Umbonal region gibbous, defined anteriorly by the broad 

 sulcus, and on the anterior side sloping abruptly to the wing. 



Anterior wing short, defined by a deep sulcus and a marked byssal sinus. 

 Posterior wing large, depressed-convex, much extended, joining the body of 

 the shell below the middle, and defined only hy the recurving of the striae ; 

 margin concave ; extremity acute. 



Test thick, marked by numerous fine stria3 of growth, which at intervals 

 are crowded into fascicles, producing an undulating surface. The striae are 

 more closely arranged, and become lamellose on the anterior part of the 

 shell. On the posterior wing the striae are regular, and at distant intervals 

 a single stria becomes sharply elevated. 



Interior unknown. 



The specimen of this species described has a length 60 mm., height 66 

 mm., and hinge-line equal to, or greater than the length of the shell. 



This species resembles G. eredum, but appears to be a more robust form ; the 

 shell is more orbicular, the umbonal region more gibbous, the surface more 

 rugose from the undulations of the fascicles of strias, and the limitation 

 between the body and the posterior wing is less strongly defined. 



Formation and localities. In the limestone of the Upper Helderberg group at 

 the Mis of the Ohio, near Louisville, Kentucky, and in Clarke county, Indiana. 



