LAMELLIBRANCHIA TA. 
157 
Glyptodesma occidentale. 
PLATE XV, FIG. 12; and PLATE LXXXVI, PIG. 9. 
Glyptodesma occidentale , Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 15, fig. 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 by the recurving of the striae; 
margin concave; extremity acute. 
Test thick, marked by numerous fine striae 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. erection, 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 striae, 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 falls of the Ohio, near Louisville, Kentucky, and in Clarke county, Indiana. 
