536 THE PALEONTOLOGY OF MINNESOTA. 



[Actinomya sutcarlnata. 



Formation and locality.— 1.0X19 er part of the Trenton formation at Beloit and Mineral Point, Wis- 

 consin. Not yet known to have been found in Minnesota, but there is no reason why it should not 

 occur in the limestone at Miuneapolis, St. Paul and elsewhere in the state. 



Mus. Beg. No. 8341. 



Actinomya subcarinata, n. «p. 



PLATE XXXVI, FIGS. 17 and 18. 



Shell of the same general form as A. modioliformis M. and W. sp., only smaller, 

 not so oblique, subalate and higher posteriorly and consequently not so elongate. The 

 hinge also is longer, the posterior margin more erect and the junction between them 

 subangular. The postero-basal margin, furthermore, is not so sharply rounded, while 

 the beaks are less incurved and farther apart. Surface of cast entirely smooth except 

 between the umbonal ridge and the postero-cardinal border, where a number of very 

 fine radiating striae are obscurely visible. 



This species reminds somewhat of the New York Trenton Modiolopsisavicu- 

 loides of Hall (Pal. N. T., vol. i, p. 161; 1847), but I cannot believe they- are id^Atical, 

 Indeed, it is more likely that they will prove widely distinct. I know of no Minne- 

 sota species with which it might be confounded. 



Formation and locality.— B,a,Te in the shaly part of the middle Galena of Goodhue county, Minnesota. 



Genus ORTHODESMA, Hall and Whitfield. 



Orthodesma, Hali. and Whitfield, 1875. Pal. Ohio, vol. ii. p. 93. 



Shell elongate, usually increasing slightly in hight posteriorly. Anterior end 

 comparatively long, contracted in front of the beaks. Valves moderately convex, 

 usually with a strong umbonal ridge and a broad mesial depression in front of it, 

 their edges fitting tightly along the .straight or sinuate ventral margin, but leaving 

 a narrow gape at each end. Umbones prominent, wide, compressed, often extending 

 posteriorly as low cardinal ridges between which the hinge is sunken. Hinge plate 

 edentulous, very thin, long, extending in almost a straight line from the posterior 

 cardinal angle, past the beaks, nearly to the anterior extremity of the shell. Liga- 

 ment linear, internal and external, the latter chiefly. Posterior muscular scar large, 

 very faint, elongate ovate; anterior scar large, though scarcely half the size of the 

 posterior, well defined, ovate or approaching semicircular in shape, the vertical 

 diameter the longest. Pallial line simple. Shells thin, marked externally with 

 more or less distinct concentric striae and wrinkles. 



Type: Orthodesma rectum Hall and Whitfield. 



The above diagnosis does not agree exactly with Hall and Whitfield's original 

 description of the genus, but as it corresponds with the fossils no apology is neces- 



