48 
BULLETIN OF THE LABORATORIES 
tiiiued across the visceral portion. Just within the cardinal border is 
a range of four or five small tubes on either side the beak. The beak 
is moderately obtuse, and passes a little beyond the cardinal border. 
The surface is covered with longitudinal rounded ribs, which, at from 
about one-fourth of the distance from the back, preserve nearly an 
uniform width to the front margin. Some of the ribs bifurcate near 
the beak, and then continue without further division, the spaces be- 
tween them being occasionally supplied with new ones; they are 
nearly straight on the back of the shell, on the sides they are curved 
toward the lateral borders, and rendered flexuous by the folds. At 
ten lines from the beak there are fifteen ribs in the space of five lines, 
the whole number is from 150 to 160. The surface is studded with 
slender tubes, which in some specimens are separated from each other 
by pretty regular intervals, and arranged in oblique line across the 
shell; in others they are scattered promiscuously over the surface. 
Ventral valve concave, visceral portion nearly plane, sub- 
quadrilateral, with several folds which continue across the shell ; these 
are prominent on the ears and side, but become obscure as they ap- 
proach the middle. 
ProdlLCtuS JJUnctatuS, Martin. 
(Plate II, Fig. 29. ) 
This is also a widely distributed species which is not very common 
at Flint Ridge. Attaining a large size; shell thin, varying from sub- 
quadrate to longitudinally subovate, sometimes wider than long; hinge 
line always shorter than greatest width of valves ; anterior outline reg- 
ularly rounded or slightly sinnous in the middle. Ventral valve gib- 
bous with a merial flattening or depression extending from the front 
nearly to the beak. Ears compressed, small (the specimen figured is 
distorted by pressure). Dorsal valve somewhat concave, with a slight 
medial prominence. Surface marked by regular, rather conspicuous, 
distant, concentric ridges, supporting numerous appressed spines. 
Length, 2.50, breadth, somewhat less. 
• 
Productus lon^ispifius , Sowerby. 
(Plate II, Figs. 25, 27, 28.) 
'Fhis very common and characteristic shell is one of the most 
