wards the posterior margin : within pearly white, iridesceKt 

 before : anterior accessonj cicatrix confluent : posterior 

 accessory cicatrix small, hardly distinct, rounded. 



SYI^'ONYMS. 



U. APicuLATUs, Nohis^ ^^ Disseminator^'' \S9,9. 

 U. ASPER, Amer. Philos. Trans. 1832. 



OBSERVATIONS. 



I obtained a few single valves of this handsome species^ 

 from a large quantity, consisting of many wagon loads, of 

 the Gnathodon truncatus, Lam. (Cyrena) at New-Orleans, 

 during a short sojourn in that city with Mr. Maclure in 1 8- 

 .27. This vast heap of dead and bleached shells, had been 

 collected on the shore of Lake Ponchartrain, and conveyed 

 by means of small vessels through the canal, for the pur- 

 pose of covering part of the " levee" of that city, as a sub- 

 stitute for a pavement. I have subsequently received se- 

 veral good specimens from my friend I\Ir. Barabino. 



In general outline and form of the disc, it has a very 

 close resemblance to U. quadrulus, Raf., but the surface 

 is in every part ornamented with crouded, elevated, more 

 or less rounded tubercles, not very different in point of 

 size, and symmetrically arranged in angulated and arqua- 

 ted series. Notwithstanding these differential traits, 1 

 think it not improbable that when we shall become bet- 

 ter ac(iuaintcd with this Protean genus, the apiculatus, 

 may be considered as a variety only of the quadrulus. 



PI. 52. 



