18 BULLETIN OF THE BUREAU OF FISHERIES. 
another. Nearly all of the mussels mentioned will occasionally reach a factory along 
with other shells. The little rainbow shell (L. iris) is commonly mistaken by shellers 
for a young mucket: therefore we have mentioned iris in connection with the mucket 
in the hope of aiding fishermen to make the proper distinction. 
One can not grasp the significance of scientific names or appreciate the sense of 
scientific classification unless the remarkable fact of convergence is understood. This 
means that species of shells not closely related may yet take the same form in certain 
characters, as in the form and color of the shell. Let us take, for example, two species 
of mussels which are so much alike in external appearance that the novice can scarcely 
distinguish between them. Both are much compressed from side to side, possess very 
thin shells, and are commonly known as paper-shells. One of these, known to science 
as Lampsilis gracilis, seems to be nearly related botlt to the mucket and to the inflated 
pocketbook shell, Lampsilis ventricosa. ‘The other compressed species, which is Proptera 
levissima, is closely related to another pocketbook type of shell, the much-inflated 
Proptera capax. Inall features of body structure and of life history, the two paper-shells 
possess relatively little in common, and the same may be said of the two pocketbooks. 
These are cases where appearances are deceiving not only to fishermen but to anyone who 
looks upon only external characters. A very obvious case of convergence is the familiar 
one of the pink heel-splitter Lampsilis alata and the white heel-splitter Symphynota 
complanata. In this case the resemblance is only on the outside, and a sheller will 
readily see the difference on the inner surface from the very distinct character of hinge 
and teeth in the two species (Pl. XXII). 
It will materially simplify our discussion to dispose of paper-shells together and 
pocketbooks together, ete. Therefore, the commercial grouping is followed, but, as 
previously suggested, the deviation from scientific classification is not generally so wide 
as might be expected. 
In the following pages the quality and distribution of the more common commercial 
species are considered.* The list immediately below indicates the relations of the various 
species, showing the order in which the different species are considered, and gives ref- 
erences to plates on which they are represented and to pages on which they are dis- 
cussed. In the list the use of brackets indicates that a species is discussed, but is not 
regarded at present as of commercial value. 
ORDER IN WHICH DIFFERENT SPECIES AND THEIR RELATIONS ARE CONSIDERED, WITH THE PLATE NuM- 
BERS AND PAGES. 
Class, group, and common name. Scientific name. Plate. Page. 
The Quadrula class: 
‘The niggerhead group— 
Niggerhead .'. ¢..fajc <a .cc cis nla caltige sletelelatn'siexinsju sin ele Qusdrula ehentsy ce srecicincsietehiesraiseicraciele ony, : 20 
Quadrula solidas 2. ii j. 155. eid om cles 4 res 22 
Tong SOG... s » steiss «0's uislelaln s/clelo ntl. qeleisis)sieiaivlo'tis Quadrula subrotunda............-..+02+0-5 sae Bree 22 
Bickory-tt. i020. k aaa cab ote ce wel Obovaria ellipsis. ..... Hie ée- ook 22 
Obovaria circulus.... 22 
[Golf-stick)..,..G24..20 ddbkecacirable totes ebitdde [Obovaria retusa].... 22 
The pimple-back group— 
Pittinle-Dack. » ca cic ccscierscaiakicia stencil aaa Quadritla pustitlosa. 0.1... c ccc c cnet ewenceenee 23 
Quadrula pustulata. .. . ee 23 
Quadrula cooperiana. . 23 
BIA DIE LORY. Secon’ s.s\cifslasreiniale sislsiainislets ebipiaieialatanisiate Quadrula lachrymosa. Se ae 23 
Quadrula fragosa........ Sond 24 
Mortkey-face 2002... Osc Rie ania soos Quadrula metanevra. ..........00.eeeeee cere eens 24 
[Rabbit's f000) os. cewrctuaac cae cctienessinee eae [Quadrula cylindrica)... .......ceeeeeee eee eee es 24 
aIn the list and following descriptive account of commercial mussels, the scientific nomenclacture of Simpson is followed. 
