Orthodesma.] LAMELLIBRANCIIIATA. 517 



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 

 m literature that it stands as the most important peculiarity of the genus, indeed, as 

 the only one separating it from Orthonota, ConVad. 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 afiinity 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 families. Thus, 0. byrnesi S. A. 

 Miller, and 0. michlehoroughi Whitfield, belong to Rhytimya, a new genus that obvi- 

 ously belongs to the same family as PhoUdella, 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- 

 lopsidce. 0. suhovale 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. 1 2 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 produc6d 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 0. 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.— WiAAle third of the Trenton shales, St. Anthony Park, St. Paul, Minnesota. 



