i8 



BULI^ETIN 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 both to the mucket and to the inflated 

 pocketbook shell, Lampsilis ventricosa. The other compressed species, which is Proptera 

 losvissima, is closely related to another pocketbook type of shell, the much-inflated 

 Proptera capax. In all 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 (PI. XXII). 



It will materially simplify our discussion to dispose of paper-shells together and 

 pocketbooks together, etc. 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 coinmon 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 coinmon name. 



Scientific name. 



Plate. 



Page. 



The Quadrula class: 



The niggerhead group — 

 Niggerhead 



Long solid . . . 

 Hickory-nut . 



[Golf-stick] 



The pimple-back group — 

 Pimple-back 



Maple-leaf. 



Monkey-face. . 

 [Rabbit's foot]. 



Quadrula ebenus 



Quadrula solida 



Quadrula subrotunda . 



Obovaria ellipsis 



Obovaria cir cuius 



[Obovaria retusa] 



Quadrula pustulosa . . 

 Quadrula pustulata . . 

 Quadrula cooperiana . 

 Quadrula lachryraosa . 



Quadrula fragosa 



Quadrula metanevra . 

 [Quadrula cylindrical . 



II. III. 



II 



IV. ... 

 III.... 



V... 

 VI.. 

 VI.. 

 VII. 

 VII. 

 VII. 

 VII. 



23 

 23 

 23 

 23 

 24 

 24 



24 



a In the list and following descriptive account of commercial mussels, the scientific nomenclacture of Simpson is followed. 



