Species of the Genus Unio. 185 



task of describing the following shells with diffidence, although 

 confident that my observations will help to increase our know- 

 ledge of the genus, even if one or two forms should eventually 

 be decided to be varieties. I have not quite exhausted my store 

 of novelties, and am, moreover, inclined to believe that some of 

 the shells which I still regard as varieties would be pronounced 

 to be distinct species by some writers who have studied the 

 genus more extensively. 



The remarks published by Lamarck on the subject are worthy 

 of note. He says : " Ce qui se montre dans tous les genres ou 

 nos collections se sont bien enrichies, savoir, que les especes se 

 nuancent et se fondent les unes dans les autres, dans le cours 

 de leurs variations, se fait ici encore plus fortement remarquer 

 qu'ailleurs, et confirme ce que j'ai dit de Yespece dans ma ' Philo- 

 sophie Zoologique ' et autres ouvrages ; aussi la determination 

 des especes du genre Mulette est-elle tres-difficile." 



I add some notes on species already published, and which 

 have fallen under my observation. 



1. Unio Jenkinsianus, B., n. sp. 



U. testa transverse elongata, subacuminato-ovata, valde inaequilaterali, 

 tumida, crassa, antice brevi rotundata,' postice elongata, minime 

 alata, superne sensim descendente, demum angusta, obtusa ; mar- 

 gine basali antice convexiusculo, postice subrecto ; ligamento brevi ; 

 disco valde tumido, transverse plicato ; epidermide picea, versus 

 marginem ventralem olivacea ; umbonibus leviter convexis, decor- 

 ticatis, apicibus obtusis ; dentibus arcuatim sitis, crassiusculis, 

 erosis, cardinalibus elongatiusculis, valvae sinistrae subsimplicibus, 

 dextrse duplicibus laminatis, lateralibus remotis subelongatis, valvse 

 sinistrse duplicibus, lamina interna demum incrassata : margarita 

 violaceo-albida, iridescente. 



Long. 52, lat. 94, diam. 38 mill. 



Habitat in fluvio Assamensi Berhampooter dicto. 



The anterior cicatrices are widely separated, the posterior 

 confluent, and in the right valve a distinct cicatrix is placed in 

 the angle formed by the confluent portions. The dorsal cicatrices 

 are situated between the shallow cavity of the beaks and the 

 cardinal tooth. The subapical lobe of the cardinal tooth in the 

 right valve is obsolete, the other somewhat thickened, oblique, 

 and erect. In the left valve the cardinal tooth is double and 

 oblique, the internal portion being thickened. The lateral teeth 

 are oblique, erose, and rather short, especially the double one in 

 the left valve, in which the lower lamina is very thick, especially 

 towards the extremity. The ligamental slope is flattened, not 

 alate as in U. marginalis, and the ligament is very short when 

 compared with that of the different varieties of the latter species. 



The very tumid form, the sloping posterior end, the absence 



