520 THE PALEONTOLOGY OF MINNESOTA. 
[Orthodesma canaliculatum. 
ORTHODESMA CANALICULATUM. 
PLATE XXXVII, FIGS. 7—11. 
Shell elongate, the length three times the hight; cardinal and basal margins 
straight, nearly parallel; posterior margin oblique, rounding into the hinge line, below 
which it slopes backward with a gentle curve to the postero-basal extremity where 
it turns abruptly into the basal line; anterior end contracted in front of the beaks, of 
moderate length, rounded, most prominent a little above the middle. In a side view 
the beaks project very little, are compressed by a broad shallow sulcus which crosses 
the valves and occupies a large part of the anterior three-fifths of the shell; umbonal 
ridge rather distinct, extending from the beaks to the postero-basal extremity. In 
a cardinal view of casts of the interior, the only condition in which the species has 
been noticed, the hinge line is strongly depressed, lying at the bottom of a wide and 
deep channel, deepest between the rather widely separated beaks and gradually 
shallowing posteriorly. Casts usually almost smooth, exhibiting only a small num- 
ber of obscure concentric furrows. One specimen preserves a small part of the shell 
and this shows that near the dorsal edge the outer surface is marked with somewhat 
regular raised lines, about six of them in 5 mm. The best preserved casts exhibit in 
the posterior half of the mesial sulcus a number of obscure radii. Anterior muscular 
scar sharply defined at the inner side, rather small, broad-oval or circular, occupying | 
the middle two-fourths of the upper half of the anterior end. Posterior impression 
somewhat larger than the anterior, subcircular, with a narrow prolongation extend- 
ing forward nearly parallel with the posterior cardinal margin. Pallial line distinct 
in the anterior half, consisting (on the casts) of a straight row of obscure pustules 
extending in a slightly oblique direction from the base of the anterior adductor 
impression toward a point much wearer the ventral border. 
There are several peculiar features about this species. (1) I have never seen its 
valves separate, a fact indicating, if it is not fully accounted for by the next circum- 
stance, a strong ligament. (2) Its natural position seems to have been with the 
anterior end down, and so it is commonly found in the shales, and in consequence 
it is often greatly shortened by pressure. (8) The channel-like depression of the 
hinge; and (4) the unusual course of the anterior half of the pallial line. These - 
peculiarities distinguish the species readily from all others of the genus known. 
Formation and locality—Hudson River group, Spring Valley, Minnesota. Fragments have been 
found at many localities in Ohio and Indiana where the upper beds of the Cincinnati formation are 
exposed, Good specimens, however, are very rare. 
