Contributions to Palaeontology. 



105 



"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 towards the beak of the ventral 

 " valve, which it overhangs and embraces as with a 

 " deltidium. Surface marked with fine concentric striae : 

 " when exfoliated, longitudinal striae can also be detected 

 " towards the circumference of the shell. Length f of 

 11 an inch ; greatest width, of an inch. 



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

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



The description above applies to the spatulate forms with elongate 

 slender apices, which are common at the Falls of St. Croix. Asso- 

 ciated 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 Lingula 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 impressions in the two are considerably differ- 

 ent ; 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 

 afford little means of comparison ; that of the shorter valve 

 [Trans. v.~] 14 



