298 CONRADS SYNOPSIS OF THE GENUS PLEIODON. 



Localit.ij, Wabash river. Mr. Poulson's cabinet. 



This species has some affinity with U. lenior, Lea, but the male has probably been 

 confounded with that of JJ. toridosus, Raf. The female, however, has no resemblance 

 to that of the latter, and the shell is without tubercles, 



U. ALTUS. PI, 27, fig. 5.-- -Transversely ovate, elevated, thick, with a posterior depression cr furrow, 

 moderately impressed in the adult, scarcely visible in the young ; anterior margin very obliquely 

 truncated, posterior margin direct, obtusely rounded j epidermis ochraceus, wrinkled inferiorly, marked 

 over the umbo with interrupted rays, some of them broad ; within white; cardinal teeth robust, direct; 

 lateral teeth very oblique, slightly curved, double in each valve. 



Locality. Cabinet of the Academy. 



This species came from one of the Western States, probably Tennessee. There 

 are but two specimens in the Academy's collection, of which the figure represents 

 the youngest. The adult is more produced or angulated at the posterior extremity, 

 and more distinctly depressed on the posterior side. It measures Ig inches in length 

 and 2 inches in height. 



This species has some resemblance to U. macnlatus, but is more elevated, and the 

 young shells of the two species are widely different. The markings resemble 

 those of JJ . personatus. Say, but it cannot be confounded with that species. 



U, VELATUS. PI, 27, fig. 0.— Ovate-cuneate ; disks flattened or slightly ccntractcd; beaks eroded, distant 

 from the anterior extremity, which is regularly rounded ; posterior end obtuse, the extremity nearly on 

 a line with the ba?e and subangulated; hinge margin declining; umbonal slope subangulated; basal 

 margin straight in the middle ; epidermis olivc-ycUow with dark green filiform rays, and posteriorly 

 and on the umbo broad interrupted rays; within white; cardinal teeth nearly direct, rather small and 

 robust ; lateral teeth moderately thick. 



Locality. River St. Fois. Cabinet of John S. Phillips. 



Allied to TJ. Foremanianus, Lea, but proportionally longer, thinner, and with very 

 different lateral teeth, which are much thicker in the allied species. 



Synopsis of the Genus Pleiodon. 

 By T. A. Conrad. 



1. P. OVATA. 



Iridina ovata, Swains. 

 Shell elliptical, very uniquilateral, convex, posterior side ouaeiform; cavity of umbo small; posterior 

 muscular impressions confluent. 



2. P. Maomubtrei, Con. Journ. Ac. Nat. Sci. vol. vi. 178, 13. Ovate, ventricose, cavity of umbo 



profound ; line of the anterior cardinal teeth suddenly contracted over the muscular impression ; posterior 

 muscular impressions not confluent. 



