SWALLOW DESCRIPTION OF NEW FOSSILS. 81 



Descriptions of some New Fossils from the Carboniferous 

 and Devonian Packs of Missouri. 



By G. C. Swallow. 



[Read October 16, I860.] 

 Pentremites Missouriensis. 



Body large, ovate, convex below; base moderately convex : 

 basal plates form a pentagonal convex disk; the three sides, 

 formed by the union of two adjoining plates, are concave: 

 nul 'oil plates deeply forked, with a high sharp, edge bordering 

 the poral plates: interradial plates lanceolate, the two lower 

 sides sigmoid; apex not so high as the convex summit of the 

 body : pseudam bulacral areas long; the two sides slightly 

 convex, and sloping to the depressed central line. All the 

 sutures, formed by the junction of the radial plates with each 

 other, with the interradial plates, and with the basal plates, 

 are bordered by fine stria?, and a distinct depressed line on 

 each side of the suture and parallel to the edges of the plates. 



This fossil most resembles the P. Cherokeeus of Prof. 

 Troost ; but it is longer, more ovate; the interradial plates 

 are longer and their points not so high, and their lower mar- 

 gins convex or sigmoid. But the distinguishing characteris- 

 tics are the striae and depressed lines parallel to the sutures, 

 as above named. 



This rare and beautiful Pentremites is found associated 

 with the P. Cherokeeus in the Archimedes Limestones of 

 Missouri and Illinois. 



Orthis Clarkensis. 



Shell of medium size, transverse, subelliptical. Ventral 

 valve convex near the beak, depressed in front, flattened to- 

 wards the lateral margins; beak small, moderately elevated ; 

 area shorter than the width of the shell. Dorsal valve more 

 convex, with abroad sinus on the middle of the anterior por- 

 tion. Surface marked with numerous, small, rounded, tubu- 

 lar, radiating costse (increased by implantation), and by fine 

 concentric striae, and by lines of growth more or less con- 

 picuous. 



It resembles the 0. Swallovii of Prof. Hall, but its form 

 and concentric striye mark it as distinct. 



This shell is abundant in the lower beds of the Keokuk 

 Limestone, associated with 0. Keokuk, at various localities»in 

 Iowa and Missouri. 



C 



