1854.] 171 



3. I. salebrosus. Shell semicircular, with oblique concentric ribs; ribs remote, 

 irregular, undulating, acute. 



Distinguished from the other species by the coarse, sharp ribs. 

 Locality, Cahawba, Alabama. 



4. 7. in flatus. Shell semicircular, somewhat equivalve, surface undulating; ribs 

 oblique, very irregular, almost obsolete; beaks distant, buccal side inflated; 

 anal side compressed, with a broad shallow channel in each valve. 



Locality, Columbus, Mississippi. Dr. Spillman. 



5. I. triangularis. Shell somewhat trigonal, nearly'equilateral ; with concen- 

 tric transverse undulations or ribs, buccal side somewhat carinated ; beaks 

 compressed, slightly recurved. 



Locality, Columbus, Mississippi. Dr. Spillman. 



6. I. proximus. Shell compressed, inequilateral ; ribs concentric, regularly 

 curved, approximating. 



The closely arranged ribs characterises this species. 

 Locality, Columbus, Mississippi. Dr. Spillman. 



Ostrea. 



1. O. crenulata. Shell subfalcate, lower valve with closely plaited squamose 

 ridges ; upper valve convex, with smooth plates ; ligament area long, triangu- 

 lar; margin crenulated. 



Resembles the young of O. sellteformis, and having often the habit of O. 

 equestris, being attached to branches of corals, etc., by the entire length of the 

 lower valve. 



Length 1.5 in. 



Locality, Eufaula, Alabama. Mr. Thornton. 



RUDISTES. 



Mr. Lyell was the first to point out the existence of this curious family, in 

 the cretaceous rocks of the United Slates. 



The fragments then known were referred to the genus Hippurites. From that 

 time to the present I have endeavored, with the assistance of many friends, to 

 collect the fragments of Rudistes, scattered over the prairie region of Alabama 

 and Mississippi. The result has been the determination of two genera and 

 seven species, and, what is remarkable, not a single fragment that could be rer 

 ferred to the well marked genus Hippurites has as yet been found. 



Without figures it is not easy to give characteristics of the species of a 

 family, the remains of which are so badly preserved. The following, however, 

 may serve for the present. 



Radiolites, Lam. 

 Sphcerulutes, Delam. 



1. R. Ormondii. Shell, lower valve comparatively thin, long, lamellae angular; 

 lip turned upwards at the outer circumference; ramifications of the mantle 

 slightly impressed on the lip; cells small; outer surface ornamented with verti- 

 cal ridges composed of the zigzag edges of the leaves composing the shell ; 

 inner surface smooth, with a thin plate extending a short distance into the aper- 

 ture. 



Distinguished by the thinness of the shell and ornamented exterior. The 

 fluting and apparent joints of the outer surface suggest, at first sight, a frag- 

 ment of a fossil plant. 



Inscribed to my friend, Dr. John Ormond, of Tuscaloosa, who discovered this 

 interesting species. 



Locality, Marengo county, Alabama. 



2. R. lamellosis. Shell, lower valve thick, conical, large ; lip nearly horizon- 

 tal, or slightly depressed towards the circumference, with radiating smooth 

 ridges, more numerous towards the outer circumference; outer edge slightly and 



