410 Mr. Barnes ow the Genera Unio 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 witha 
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 Ais 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 =4°133955 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 124 
Millim. to half an inch, and 4 inches to 100 Millimetres, 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- 
