128 SIXTEENTH KEPORT ON THE CABINET OF NAT. HISTORY. 



satisfy myself that either L. prima or L. antiqua has been found in the 

 Mississippi valley. The species of Lingula present so few salient charac- 

 ters, that it is not surprising that cursory examinations of these fossils, as 

 they are crowded together in great numbers in the beds at St. Croix falls, 

 or when scattered in the sandstone, poorly preserved and often distorted, 

 should sometimes lead to erroneous determinations, which can only be cor- 

 rected by longer continued examinations and comparisons. 



Dr. Owen has described, under the name Lingula jnnnaformis, 

 the spatuliform Lingula from the St. Croix falls, as follows : 



" Specific character. Shell shaped much like a Pinna, moderately 

 " convex, expanded and running to a point at the beak, which, 

 " in the dorsal valve, is long, conical and slender, curved to- 

 " wards the beak of the ventral valve, which it ovarhangs and 

 " embraces as with a deltidium. Surface marked with fine con- 

 " centric stride : when exfoliated, longitudinal striae can also 

 " be .detected towards the circumference of the shell. Length 

 " |- of an inch ; greatest width, -^\ of an inch. 



" This fossil is abundant in the silico-calcareous layers near the base of 

 " member 6 of F 1 at the Falls of the St. Croix, Minnesota." 



The description above applies to the spatulate forms with elongate slen- 

 der apices, which are common at the Falls of St. Croix. Associated with 

 these, and in about equal numbers, there is a broad and much shorter form 

 with rounded beak, the margin of which is a little concave from the inner 

 side (as seen from the extremity), apparently for the passage of a peduncle. 

 Among hundreds (and even thousands) of individuals examined, I have 

 never seen two valves of this or the other form in their natural relations. 

 Of both forms, the valves are always separated. 



The description of Dr. Owen does not define the two valves ; 

 and since he has recognized Orbicula and other species of Lin- 

 gula in the same beds, I have been in doubt regarding his views 

 of these two forms and their relations to each other. 



In order if possible to decide their relations, I have had the 

 shell removed from several individuals, that the casts might be 

 studied. By this process, I have decided that the muscular im- 

 pressions in the two are considerably different ; but they may 

 pertain to the dorsal and ventral valves of a species, while at the 

 same time their characters do not furnish positive evidence that 

 they are not of distinct species. 



The muscular impressions are so unlike Lingula as to aff'ord 

 little means of comparison ; that of the shorter valve resembling 

 the flabelliform muscular impressions of the ventral valves of 



