AND DESCRIPTIONS OP NEW SPECIES. 65 



into the mass of its thick beaks to the distance of one third of an inch, 

 where di false beak is exhibited covered with a thin layer of epidermal 

 matter, and a fracture of this false beak displays another within, 

 entirely covered with the same matter. In the same specimen, which 

 is more eroded than any I have seen, a considerable portion of the 

 cardinal tooth is visible from the exterior, and this portion is also 

 covered with epidermal matter. 



I will take the opportunity here to remark on the absolute necessity 

 of studying the different ages of the species of this family, to enable us 

 to decide upon new species. I have never been more thoroughly 

 convinced of any thing than this. When I have been able to do so, I 

 have always placed in my cabinet at least three or four specimens of 

 different ages. Four years since, I obtained a large old specimen of 

 the Unio multiplicatus (herein described), which I placed with my 

 specimens of Unio plicatus. Two young specimens were received at 

 the same time, and so totally different were they in appearance, that 

 it did not occur to me there was any similarity between them and the 

 old one. These were placed with the Unio tubcrculatus, in the belief 

 of their being a compressed variety of that species. A specimen 

 subsequently received, which furnishes the engraving, proved at once 

 their identity by displaying the numerous undulations in the region of 

 the beaks. 



I have continued to give my attention to the habits of the animals 

 of this family, but I have in vain attempted to satisfy myself as to the 

 nature of their food. Dissatisfied with the results of the observations 

 mentioned in volume third, I procured, among other species, a fine 

 Unio cariosus, the valves of which were much more gaping than usual. 

 Selected specimens of various species were placed in a glass vase, in 

 the bottom of which was placed clean white sand, so that their natural 

 beds might be somewhat imitated. In this vessel they assumed their 

 natural position by pushing the sand behind them with the protruded 

 foot, thus forming a pit into which the base of the shell gradually fell, 

 the ligament taking the most elevated situation. In this position they 

 soon began to travel round the vessel, and this locomotion continued 

 for some days, when it ceased entirely. 



Their extreme timidity or apprehension on the approach of danger 

 Vol. IV.— R 



