Abnormal Fresh-water Shells. 55 



Unto complanatus, Solander. 

 Plate II, figs. 2-6. 



Figure 2 represents a specimen similar to the preceding, but with 

 more numerous and stronger vertical plications. The shell in these 

 species is normally smooth and we must seek for an explanation of the 

 cause of the plications in the soft parts of the animal, as they are evi- 

 dently not due to accidental causes. From the examination of a num- 

 ber of individuals presenting these plications in various degrees of 

 prominence, and from the inspection of the living animal, it is evi- 

 dent that these abnormal features are produced by the rapid growth of 

 the shell over the gills while they are distended with fry. Unio os- 

 oecMi, a species from China, is classed with the plicate forms in Lea's 

 Synopsis of the TJnionidce, but the plications do not seem to be a con- 

 stant characteristic of the species. The plications are not always 

 present and, when they do occur, they are usually obscure and similar 

 in position and expression to those specimens of U. nasutus and U. 

 complanatus here presented and probably have a like significance. 



Figure 3 represents the right valve of a specimen modified by acci- 

 dental deformity. The umbo is nearly central, and the upper anterior 

 portion of the valve is flattened and deeply sulcate. 



The next specimen (figs. 4, 5) is a very elongate cylindrical form 

 with an excessively thickened pallial margin. 



The last individual to be noted (fig. 6) is an apparently normal form, 

 as no marks of accidental or natural deformity can be detected. It 

 was found associated with numerous specimens of U. complanatus, 

 and is here referred to this species, although seemingly presenting 

 marked specific differences. The outline is regularly elliptical, and 

 the prominent beak is situated just anterior to the middle of the 

 length. The cardinal teeth are elongate, and the lateral tooth is 

 short and oblique — characters which do not belong to U. complanatus. 

 Should it ultimately prove of a distinct species, it would be of a form 

 hitherto unknown to this locality. 



Specimens similar to the preceding briefly noted forms are often 

 overlooked or considered as unimportant by many collectors ; but to 

 a student of morphological variation and possible specific change, they 

 are extremely interesting. After numerous accidental and natural 

 changes have been illustrated and described, embracing many genera 

 and species, it will be possible to generalize important biological facts 

 relating to the classification of species and manner of growth of the 

 organisms. 



