110 Mr. Barnes on the Genera Unto and Alasmodonta. 



M. Lamarck has confessed the great difficulty of deter- 

 mining the species of the genus Unio on account of their 

 " shading and melting into each other in the course of their 

 variations." This difficulty is surely not obviated by short 

 and equivocal descriptions. Short definitions may have an 

 appearance of scientific neatness, but their brevity is an in- 

 superable obstacle to a learner, especially when, as it com- 

 monly happens, the same terms are applied to different spe- 

 cies. M. Lamarck applies the term ovate, either by itself or 

 compounded with another word, to the description of thirty- 

 two, out of his forty-eight species. Now it will be apparent 

 to every one that, as this is made a leading feature in his 

 descriptions, it must be the cause of endless perplexity to 

 the unlearned, and of constant uncertainty even to the ex- 

 perienced. For the purpose of discrimination it is useless, 

 and might almost as well have been omitted, unless it had 

 been placed at the head of a section. 



M. Lamarck dwells most on the external form, and with a 

 great latitude of compound epithets, he has not succeeded 

 in making his descriptions intelligible, without danger of mis- 

 take to those who have not seen his specimens. Ten or 

 twelve latin words cannot so describe a Unio as to identify it, 

 and distinguish it from all others. We have therefore adopt- 

 ed full descriptions, the obvious utility of which needs no 

 comment. If short definitions are insufficient, full descrip- 

 tions become absolutely necessary. M. Lamarck, general- 

 ly mentions the breadth of shells in Millimetres, which we 

 have reduced to inches and lines, or what is the same thing, 

 to inches and decimals. The multiplier '039371, which mul- 

 tiplied by any number of Millimetres gives the correspond- 

 ing English expression, as, Unio Crassidens 105 Millim. 

 105 X -039371 =4133955 or four inches and 1 line. Di- 

 viding the English inches by the multiplier, will reduce Mr. 

 Say^s measures to M. Lamarck's by which means they may 

 be more readily compared. For ordinary purposes 12^ 

 Millim. to half an inch, and 4 inches to 100 MiUimetres, will 

 be sufficiently exact. 



But the breadth, or as Lamarck often says the " apparent 

 length''^ of the shell is nearly useless without the length ; for 

 two shells may be of the same breadth, and yet differ total- 

 ly in their other dimensions. For instance, the U. Crassus 

 and U. Nasutus may each be 26 lines broad; but the Cras- 



