ATHYRIS OF THE UPPER HELDERBERG AND HAMILTON GROUP. ^§9 
Atliyris vittata. 
PLATE XLVI : Figs. 1-4. 
Athyris vittata : Hall, Thirteenth Report on the State Cabinet, p. 89. 1860. 
Compare Athyris concentrica, A. spiriferoides , etc. 
Shell ovate-subquadrate, gibbous, with the mesial fold and sinus distinct; 
front conspicuously sinuate : hinge-line short : cardinal extremities 
rounded. 
Ventral valve gibbous above, more convex than the dorsal; umbo pro¬ 
minent ; the beak incurved and truncated in the plane of the longitu¬ 
dinal axis by a round foramen, curving very abruptly to the cardinal 
and cardino-lateral margins; the centre marked by a well-defined 
mesial sinus, which is continued nearly or quite to the beak, and 
becoming much deeper and subangularly margined towards the front. 
Dorsal valve a little less gibbous than the ventral, sides regularly curv¬ 
ing; the middle of the upper part distinctly prominent, and developed 
below in a strong mesial fold which is abruptly elevated in front. 
Surface marked by regularly imbricating lamellose lines of growth, 
which, on the better preserved surfaces, are finely crenulate on their 
edges, and the intermediate spaces striate. 
Interiorly the spires of this form, in their first volution and in the 
accessary lamellse, are quite distinct from those of A. spirif eroides. The 
accompanying figure is an illustration of the first volution of the spires, 
with the accessary lamellse. 
Athyris vittata. 
[ Paleontology IV.] 
