LAMELLIBRANCHIATA. 517 
Orthodesma.] 
sary. They make, for instance, the erroneous statement that the hinge plate is bent 
down in front of the beaks; and the fictitious feature has become so well established 
in literature that it stands as the most important peculiarity of the genus, indeed, as 
the only one separating it from Orthonota, Conrad. Now, despite the fact that the 
hinge plate is nearly or quite as straight in Orthodesma as in Orthonota, I am fully 
satisfied that there is little affinity between the two genera. The Lower Silurian 
genus, doubtless, is closely related to Modiolopsis and Actinomya. Not so, however, 
with the Devonian genus, which seems to me to be totally different and nearer Solen 
than Modiolopsis. 
Species have been placed under Orthodesma that are very different from the 
types, some of them belonging, I believe, to other fathilies. Thus, 0. byrnesi S. A. 
Miller, and O. mickleboroughi Whitfield, belong to Rhytimya, a new genus that obvi- 
ously belongs to the same family as Pholadella, Hall, and Allorisma, King. 0. cunei- 
forme Miller, has recently been made the type of his new genus Sphenolium. This 
‘genus seems to be related to Cuneamya and therefore cannot belong to the Modio- 
lopside. O. subovale Ulrich, together with a number of undescribed species, belongs to 
the new genus Psiloconcha, while 0. saffordi Ulrich, should be referred to Actinomya. 
ORTHODESMA MINNESOTENSE Ulrich. 
PLATE XXXVII, FIGS. 12 and 14. 
Orthodesma minnesotense ULRICH, 1892. Nineteenth Ann. Rep. Geo. Nat. Hist. Sur. Minnesota, p. 228. 
Shell small, elongate, subrhomboidal, with the dorsal and ventral margins nearly 
straight and parallel; the length two and one-half times the width. Beaks small, 
incurved, compressed, projecting moderately above the hinge and situated about one- 
fourth of the entire length from the anterior extremity; posterior umbonal ridge 
subangular, cardinal slope abrupt, in casts of the interior with a linear impression 
close to and on each side of the hinge line. Anterior end small, contracted a little 
in front of the beaks, almost uniformly rounded; posterior end oblique, sloping 
upward and forward from the produced and narrowly rounded lower part. 
Interior with the anterior pair of muscular scars rather distinctly marked and 
large; above and between them and the beaks, two other very small pairs of scars 
are to be seen on the specimen figured above, but the posterior muscles left no 
appreciable impressions. Surface of casts with few obscure folds of growth. 
This shell is related to 0. curvatum Hall and Whitfield, though more nearly 
approaching O. contractum Hall, in its outline. It differs from both in having the 
posterior end narrower and in wanting the strong concentric furrows which occur 
on the posterior cardinal slopes of those shells. 
Formation and locality —Middle third of the Trenton shales, St. Anthony Park, St. Paul, Minnesota. 
