372 PROCEEDTXaS OE THE NATIONAL MUSEVM. 



For a species so iiiiiiierons in iiidividnals it is quite uniform iu outliue, 

 though examples are occasionally met witli that are decidedly triangu- 

 lar, and infrecjuently an extremely elongated specimen occurs — not one 

 in a hundred. 



The more ventricose individuals are oidy moderately tumid. The 

 epidermis in large iidnlts is notably thick and deciduous, soon contracts 

 and peels of!", even after a good oiling or treatment with glycerine, and 

 the coh)rs are fugitive. 



The color of the rays as given in the foregoing description, "purple- 

 violet," suggests that the specimen upon which the description is based 

 was more or less denuded, for when the epidermis is intact the rays 

 can hardly be called purple-violet, although it is possible that such 

 an exam])le may sontetimes occur. Tn half grown examples or individ- 

 uals under that si/e the epidermis is more tenacious or adherent; in 

 these, however, a slight rubbing with sweet oil or glycerine is a wise 

 precaution. 



('Onrad's description, as will be seen by what follows, is altogether 

 too brief and fails to give any idea of the range of color that is exhib- 

 ited by this species where a large number from different localities are 

 brought together. The general or ground color of the surface runs 

 from nearly white to dingy cream, to dark cream, pale umber, pale 

 purple, or both tints in the same shell, pale ochre or sienna yellow, pale 

 reddish brown to dark brown or light chocolate, with more or less 

 bluish purple, etc., with various markings upon these ground tints. 

 None of these tints or colors are what may be called brilliant; they 

 are more vivid in the young shells, as well as in fresh or recently 

 collected specimens. 



1 am not aware of any other species of Tivela that exhibits so many 

 color varieties and markings, within so narrow a range of tints, as 

 this. It re(piire^ a great number, however, and from numy localities 

 to fairly exhibit this variability. With a good series made with this 

 intention a (juite attractive result is attainable. 



The series which forms the basis for this review, made by me 

 expressly for the IJ. S. National Museum, includes selections from not 

 less than a thousand specimens. 



The first general segregation, the type aspect being in mind, is tliat 

 ornamented with rays. 



The greater proj^ortion in any large promiscuous number are plain, 

 but as a rayed example is the type of the species, the plain shells will 

 have to be classed as varieties. The rayed ones may be grouped as 

 follows : 



GROUP I. 



Dark rays on a light ground. 



First, the typical, ground tints "cream." This must be expanded 

 so as to include other ground tints, namely, dingy cream, dark cream 

 slightly tinted with umber, sometimes very pale purplish; the anterior 



