,, . . I. \MK1.I .'.IT 



OrtlMMlrMitft. ] 



sary. They make, for instance, the erroneous statement that the hinge plate in bent 

 down in front of the Leak-; ami the lictition- feature has become so well established 

 in literature that it -tan. Is as the most important peculiarity of the genus, indeed, M 

 the only one separating it from Urthanntn. Conrad. Now, despite the fact that the 

 limp- plate is nearly or unite as straight in <>,-th<tdesma as in Orthonota, I am fully 

 satisfied that there is little attinity between the two genera. The Lower Silurian 

 genus, doubtless, is closely related to Modiolopsis and Jdimaiytr Not so, however, 

 with the H.'vonian genus, which seems to me to be totally different and nearer >'.*/ 

 than M'nli>ilnjnu. 



have been placed under Orthodesmn that are very different from tin- 

 types, some of them belonging, I believe, to other families. Thus, 0. byrnesi S. A. 

 Miller, ami u. ntift;l,l,.;,n</iii Whitfield, belong to Rfii/titnya, a new genus that obvi- 

 ously belongs to the same family as Pttoladelln. Hall, ami Mlorisma, King. 0. cunei- 

 Miller, has recently been made the type of his new genus Sphenolium. This 

 (.'onus seems to be related to Cinienmya and therefore cannot belong to the Modio- 

 lliiilii. a < ; / I Irich, together with a nnmber of undescribed species, belongs to 

 tin- new genii- /'>//,v M ,-/i,/. whil,- o. snjfordi Ulrich, should be referred to Aetinomya. )/ 



ORTHODKSMA MINNESOTKNSE I 'I rich. 



PLAT! XXXVII. KIOS. 1 1 and 14. 

 Orthodetma minnrtotrn* I 1.1:11 n, 18B2. Nineteenth Ann. Rep.Oeo. Nat. Ulnt-Sur. Minnesota, p.2J8. 



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, 

 inrurvoil. compressed, projecting moderately above the hinge and situated about one- 

 fourth of the entire length from the anterior extremity; posterior urabonal ridge 

 sntiangular, 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 obliqin-. -lopin^ 

 upward ami forward from the produced ami narrowly rounded lower part 



Interior with the anterior pair of mii-cnlar - 'her distinctly marked ami 



large; above and between them and the beaks, two other very small pair- of scars 

 are to be seen on the .-peri men figured above, but the posterior muscles left no 

 appreciable impressions Surface of caste with few obscure folds of growth. 



This shell is related to 0. mrrninni Hall and \Vhitlit>l<l. though more nearly 

 approaching 0. r ,,(, i<f KDI Hall, in its outline. It ditter- from l.oth in having the 



'rior end narrower and in wanting ti mi-cntric furrow- which occur 



on the posterior cardinal -lopes of those shells. 



y. Mi . <*. 



