258 



BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



Large specimens reach a diameter of 42 mm. I have seen others, 

 containing eggs, not more than 11 mm. in diameter. 



Darwin did not give the distribution of rubescens, but his figure 

 and description apply perfectly to the California!! race defined above, 

 for which I use his name in a restricted sense. Reports of rubescens 

 from other regions were doubtless founded on other races of some- 

 what similar color or upon erroneously localized specimens. T. s. 

 rufotincta is paler, with finer pores, producing finer external sculp- 

 ture; it has a differently shaped tergum and numerous short teeth on 

 the occludent border of the scutum. It is not closely related to the 

 Californian race. 



Form elegans Darwin (pi. 61, fig. 2). Outer layer of the w r all 

 preserved, except sometimes near the summit ; white under a cream- 

 color or cream-buff epidermis, which, when perfectly preserved, bears 

 close concentric fringes of short, delicate hairs; strongly ribbed, the 

 ribs crossed by fine concentric growth-ridges. Sheath reddish in 

 some examples, and these usually show reddish spots where eroded 

 near the orifice, and if filed, a red layer is found under the white, 

 caused by the colored filling of the pores. Some individuals, however, 

 are truly albino, without red in the sheath or pores, and others show 

 only a few small red spots on the sheath. The opercular valves are 

 like those of rubescens. The largest specimens seen are 20 mm. in 

 diameter. 



The form described as var. elegans by Darwin does not seem to be 

 separable from rubescens, as there are some transitional individuals. 

 It appears to be a form, perhaps an incipient race, in which the 

 cuticle, with the underlying whitish outer layer, are persistent up to 

 a certain size, as in the Lake Worth Inlet form of squam>osa, noticed 

 on p. 255. It is not a stage passed through by all individuals of T. s. 

 rubescens, as I have seen typically eroded and colored specimens only 

 11 mm. in diameter. The exact status of elegans must be determined 

 by observations by those who have opportunity to study and collect 

 them. 



