OF THE FAMILY OF NAIADES. 419 



. - TT TJ ( V < 



lo. U. verrucosus, Bar. 

 1G. U. tubcrculatus, Bar. 



rerrucosa, Valcn. 



tuberculosa, Valcn. 



17. U. gibbosus, Bar. mucronatws. Bar. 



18. U. cuneatus*, Bar. 



19. U. ventricosusf, Bar. 



20. U. siliquoidcus, Bar. inflatus, Bar. 



21. U. triangularis, Bar. 



cornutus has three or four distinct " horns," and the varieties gradually increase 

 in the number, and vary in the forms of those elevations until they arc lost in two 

 ridges passing from the beaks to the posterior basal margin. It is exceedingly 

 interesting to trace these gradual changes of form ; and to illustrate the fact of 

 the anomalous varieties being of the same species, I have arranged forty-three 

 specimens in my cabinet, no two of which are alike. Dr Ilildrcth has made a 

 species of one of these varieties, and calls itfoliatus. It appears that Mr Barnes 

 and himself had seen only this specimen. I have had three or four in my posses- 

 sion for three years, and at first my impression was in favour of their being 

 new, but examining them with that excellent conchologist, Mr Stewart, we found 

 the line of impression, made by the mantle, did not run parallel with the deep 

 arcuation of the margin, and therefore concluded, at once, that the animal could 

 not conform to the shape of the shell, and consequently that the elongations of the 

 1 and posterior margins were unnatural. Dr II. says he is " unable to deter- 

 mine whether it is a new variety, or oidy a " lusus naturae ;" and yet he makes a 

 new species of it! ! Some of my varieties have the prolongation much more ex- 

 tended than the specimen described by Dr II. In one specimen the unnatural 

 prolongation is more than equal in extent to the natural size of the shell, designa- 

 ted by the impression of the mantle. 



* We have been murh in the habit of confounding this with the complanatut, ami 

 considering it as the analogue inhabiting the western waters. It di w- 



ever, to be retained by Barnes's name, lor il pot esses characters which the ether 

 does not. It is posteriorly more angular, and the -lull is Bubtriangular ; the com- 

 planatus is sub-rhomboidal and more carinate. • toe inhabits the western ; the other 

 the Atlantic rivers. The cuncatiis is always ponderous; the coBtplanatui, I believe, 

 never. Mr B. says his species is never rayed ; in this be is mistaken, young and 

 fine specimens have dark broad rays. 



t This is undoubtedly the species which we have known under the ntmeofglo- 

 hoxus (nndescribed). Mr B. says "it is more capacious than any of the g< nu 

 hitherto described." It resembles the otatus, but is always more globose. 



VOL. III. — 5 o 



