GINGLYMACONCHA. UNIONIDZ&. 321 
The tubes of respiration rudimentary ; the lower one fringed ; 
the orbicular muscle with a double fringe. 
The umbones in young shells are very frequently wrinkled 
or plicated. 
All the genera of this family inhabit fresh-waters, and are 
found in rivers, torrents, lakes and marshes. Linnzeus and his 
followers referred all of them to their artificial genera, Mya 
and Mytilus. 
The species of this family have particularly engaged my 
attention, from the various opinions entertained respecting 
them by different authors ; and by means of a very extended 
series collected from every part of Great Britain, I have been 
enabled to clear up all doubts as to the characters of the spe- 
cies, and to the varieties to which they are all more or less 
subject. 
I must return my warmest thanks to the following persons: 
to the Right Hon. Sir Joseph Banks; to Miss Bennet ; to 
the Rev. J. Bulwer; to J. S. Henslow, Esq., Professor of Na- 
tural History, Cambridge ; to T. Brightwell, Esq. ; and lastly 
to Dr. F. Granger, for the very fine series of growth and va- 
rieties of the species, which all of them were so good as to fur- 
nish me from the various localities. 
Genus 60. DaMaRIs. 
Shells transverse, oblong, curved, equivalve, and inequilate- 
ral; the umbones generally decorticated ; the hinge with one 
short, compressed, slightly striated cardinal tooth. 
The orbicular muscles of the mantle unite together before 
the anterior adductor muscle at their posterior extremities, 
which are loose ; the abdomen is covered by a tendinous ex- 
pansion ; the foot lamellitorm, rounded at its extremity. 
Under the cardinal ligament the valves are furnished with 
lamellee, which are extremely fine in our indigenous species, 
and often obsolete; but in some of the North American spe- 
cies these lamellee are very much developed, and lock into each 
other. 
All the species of this genus bear pearls. 
