LAMELLIBEANGHIATA. 617 



Allodesma.l 



of preservation the species is distinguished from all Silurian lamellibranchs by the 

 peculiar surface ornamentation. 



Formation and locality, — Near Cannon Falls, Minnesota, in the upper third of the Trenton shales. 



Family CYCLOCONCHIDiE, Ulrich. 



A full description of the typical genus of this family, and of several species of 

 same, will be found in vol. viii of the reports of the Geological Survey of Ohio. 



Genus ALLODESMA, n. gen. 



Modiolopsis (part.), Ulbich, 1892. Nineteenth Ann. Rep. GeoL and Nat. Hist. Sur. Minn., p. 226. 



Shell small, transversely elongate-elliptical, moderately convex; beaks anterior, 

 small, surface with concentric lines of growth. Hinge apparently with one or two 

 long posterior lateral teeth in each valve, two cardinal teeth in the right valve, and 

 only one in the left; anterior laterals short or wanting. Anterior adductor scar 

 distinct, large, ovate, margined on the inner side by a strong curved ridge extending 

 downward from the hinge at a point immediately in front of the beaks. Just above 

 th6 adductor impressions and in front of the ridge, a small pedal muscle scar. 

 Posterior adductor impression faint, larger than the anterior, of rounded form, situ- 

 ated near the middle of the posterior cardinal slope. Pallial line simple. 



Type: A (Modiolopsis) subellipticum Ulrich, 



The species upon which the genus is founded has really no relation to Modio- 

 lopsis with which I provisionally associated it. The original type gave no hint of 

 the character of the hinge, or I would never have thought of the arrangement first 

 adopted. A better specimen, recently collected, at once led to comparisons with the 

 very different genus Cycloconcha, Miller, and proved that the relations of the shell 

 were really with that genus. The only features wherein Allodesma differs from 

 Cycloconcha, so far as data now at hand will admit of judgment, are first, the more 

 elongate form of the shell; "second, the anterior position of the beaks; third, the 

 curved ridge forming the inner border of the anterior muscular scar, and fourth, the 

 shortness or entire absence of anterior lateral teeth in the hinge. These differences, 

 though certainly of generic value, are not, as it now appears, of sufficient importance 

 to exclude the new genus from the Cycloconchidce. 



Allodesma stjbellipticdm Ulrich. 



PLATE XLII, FIGS. 9-14. 



Modiolopsis subelliptica Uleich, 1892. Nineteenth Ann. Rep. Geol. and Nat, Hist. Sur. Minn., p. 226. 



Shell small, elongate-elliptical in outline, the length about twice as great as the 

 hight; ends almost equally rounded, base broadly convex, cardinal outline more 



