416 THE AMERICAN NATURALIST [Vol. LIII 



In his notes on the fauna of Bermuda, Heilprin (1889, 

 p. 187) described as new the form ''Onchidium (Onchi- 

 diella) trans-Atlanticum, " saying^of its pigmentation: 

 ' ' smoke color or dark olive, ' ' in dorsal view. Some years 

 later Pilsbry (1900, p. 503), reporting on Verrill's col- 

 lections at Bermuda, listed this species under the desig- 

 nation by which it is now known, Onchidium floridanum 

 Dall, adding, ''Its dark olive green color agrees so well 

 with the stains on the rocks that it is very inconspicuous. 

 -(A.E.y.)" 



It is possible, though not certain, from these brief de- 

 scriptions, that Heilprin and Verrill had respectively col- 

 lected representatives of the two differently colored types 

 of Onchidium which occur at Bermuda. No constant 

 morphological differences have been detected upon which 

 to base a separation of those types, one of which is in 

 nature blue-black in general aspect, the other characteris- 

 tically of a light ("smoky") olivaceous hue. Some in- 

 termediate shades do occur, but in the field a rather sharp 

 separation of the two groups is always possible. The 

 very dark form is much the more numerous. So far as 

 pigmentation is concerned, it is probable that the blue- 

 black types contain merely a heavier deposit of a sub- 

 epidermal pigment, which is chemically alike in both 

 kinds of Oncliidia. Xevcrtlioless, we are inclined to be- 

 lieve that tilt' (liiTt riMicc in pigmentation may be repre- 

 sentative of a iTiil uviit'tic difference between the two 

 types, for reasons which may briefly be stated as follows : 

 The tw^o modes of colorization are in many places exem- 

 plified side by side ; young and adults of maximal size are 

 found belonging to either group; the occurrence of the 

 pale form is independent of the seasons, and is found 

 year after yeai- in the same spots; in other places, w^here 

 blue-black indix i( luals may be very numerous, no lightly 

 pigmented individuals have ever been encountered. 

 Therefore we an' piohahly dealing, not with a temporary 

 fluctuation in color, deteimined by some metabolic cycle, 

 but with constitutionally diverse races of Onchidium; 



