40 PALAEONTOLOGY OF THE UPPER MISSOURI. 



The following species seem to be congeneric with the original type of Sedg- 

 wicliia, %az. : K costellata (= SanguinoUtes {Lejjtodoimts) costeUatus, McCoy, Brit 

 Pal. Foss. pi. 3, F, fig. 5); S. variabilis (= SanguinoUtes variabilis, McCoy, ib. fig. 

 6) ; S. truncata (= Lejjtodomus truncatus, McCoy, ib. pi. 1, K, fig. 21 and 24), and 

 S. granosa and S. topelcaensis (= Leptodomus granosus and L. topekaensis, Shumard, 

 Trans. St. Louis Acad. I, p. 207-8). 



If we are right in referring Leptodorinis irnncatiis, McCoy, to this genus, it would 

 carry back the origin of the group at least to the Upper Silurian epoch. Some 

 similar forms also occm- in the Devonian rocks, and the genus probably attained 

 its greatest development during the deposition of the Carboniferous Series. 

 Several species presenting very similar external appearances have also been de- 

 scribed under the names Myacites, Cypricardia, &c., from the Triassic and Jurassic 

 deposits ; but we have little or no knowledge of the true affinities of many of these 

 shells, and consequently cannot pretend to define, with precision, the geological 

 range of the genus Sedgwickia. 



Sedg'wickia topekaensis ? 



Leptodomus Topelcsensis, Shdmard, Trans. St. Louis Acad. Sci. I, 1858, 208. 



Shell depressed subovate, about twice as long as high, extremely thin and fragile, gibbous in the region of the 

 beaks, and along the oblique umbonal slopes. Sides flattened above, and becoming a little concave towards the 

 base in front of the middle. Dorsal border nearly horizontal, and slightly concave in outline behind the beaks, 

 where its inflected edge is margined by a rather distinct ridge ; ventral border presenting a broad semi-ovate outline, 

 excepting a very slight sinuosity just in advance of the middle — rounding up abruptly in front and more gradually 

 behind ; anterior side prominent, gibbous, and narrowly rounded below, obliquely truncated above ; posterior side 

 compressed, narrowed, and apparently subtruncate and somewhat gaping at the extremity. Lunular impression in 

 front of the beaks (in casts) moderately distinct, defined by a faintly impressed line. Beaks prominent, gibbous, 

 a little flattened, incurved, and placed between the middle and the anterior extremity, but nearer the former. 



Entire surface, in well preserved specimens, closely covered with minute granules arranged in radiating rows ; 

 and ornamented with small concentric ridges, which are most regular and distinct on the nmboues, and end 

 abruptly along an impressed line extending from the posterior side of each beak obliquely towards the postero- 

 basal margin, thus leaving the compressed postero-dorsal region comparatively smooth. (Muscular and pallia! 

 impressions unknown.) 



Length about 2 inches ; height, 1 inch ; convexky, 0.75 inch. 



Sedgwickia topelcaensis ? 

 A. Dorsal view, partly restored, one valve being distorted in the specimen. B. Side view of same. 



This shell is closely related to S. granosa [Leptodomus granosus, Shumard), but 

 seems to agree more nearly, when aU its characters are taken into consideration, 

 with the form described by him under the name Lejitodomits topelcaensis ; particularly 

 in the possession of a shallow linear sulcus extending from the back part of each 

 beak, towards the postero-basal margin. It is true, Dr. S. does not mention the 

 presence of fine granules on the surface of the species Topekaensis, while this is one 



