218 



more inequilateral, nacre pearly white. U. Merikeanus is more com 

 pressed, tip more acute, tooth in left valve deep cleft. 



Unio petrinus. T. solida, ovato-quadrata, subequilateralis, an- 

 tice acute rotundata, postice late truncata, subtus vix arcuata, prope 

 marginem dorsalem posticum plicis obliqnis minimis ornata ; umboni- 

 bus modicis, subplanulatis ; epidermide dilute flavo-viridi ; dentibus 

 cardinalibus robustis, pyramidatis, lateralibus curtis ; margarita alba.. 

 Long. 1^; alt. 1^; lat. f poll. Llanos River, Texas. Dr. T. H. 

 Webb. 



May be compared to U. Lamarckianus, Lea, (=U. Aberti, Conr.) 

 It is less compressed, apices less acute, and less angular ; the posterior 

 altitude is greater, the posterior dorsal margin being less declivous, 

 but as the posterior truncation is less decided, the length is greater 

 compared with the height ; the surface is irregular, but destitute of 

 granules or nodules. 



Unio manubitjs. T. transversa, elongata, subarcuata, subven- 

 tricosa, antice angustata, postice acuta, margine dorsali arcuato 

 in marginem posticum desinente ; disco rudi, epidermide nitido-corneo 

 induto ; dentibus cardinalibus compressis, triangularibus, ad margi- 

 nem fere parallelis ; d. lateralibus remotis, arcuatis ; margarita argen- 

 tata, talcosa. Long. 3f ; alt. 1|-; lat. 1^ poll. Chihuahua, sixty miles 

 from Camp Ringgold. By Dr. T. H. Webb. 



Allied to U. declivis, Say = geometricus, Lea, in which the posterior 

 extremity is recurved and angular, rather than decurved with round- 

 ed angles ; and the surface is dead and dusky. In our shell, the 

 anterior extremity is mora prolonged, and much more slender. It is 

 also intermediate between U. obesns, Lea, and U. camptodon, Say. 



Anodon horda. T. tenuis, transversa, elongato-ovata, ventri- 

 cosa, dilute cornea, viridi nubeculata, antice angustata, subacuta, 

 postice subtruncata, dorso excavato, declivitate umbonali obtuse 

 angulato, submarginato, umbonibus obtusissimis, pallidis, tenuiter 

 undulatis, ad trientem anteriorem sitis ; margarita argenteil. Long. 

 31; lat. l£; alt. If poll. Comanche Creek, Texas. Dr. T. H. 

 Webb. 



Remarkable for the broad, inflated, pale beaks, presenting no dis- 

 tinct apex. In size and form, it might, at first sight, be taken for U. 

 anodontoides. It approaches to A. ferruginea and A. Dunlapiana, 

 but differs in its proportions. 



Cyclas nobilis. T. magna, solida, ovato-rhomboidea, utrinque 

 truncata sed postice obliquius ; umbonibus anticis, tumidis; margine 

 dorsali utrinque arcuatim declivibus ; margine ventrali arcuato ; val- 

 vis remote et profunda sulcatis ; epidermide flavescente ; dentibus 

 cardinalibus minimis ; intus pallida. Long. \ ; lat. \ ; alt. f poll. 

 Found near San Pedro, California. Dr. Webb. 



