88 PROCEEDINGS OF THE ACADEMY OF [1881. 



first visit I collected a few specimens of Acmvea pelta and its vars., 

 and when I returned home and began to clean the specimens I was 

 ver^' much puzzled over some specimens of No. 458. SeA'eral 

 years ago I had collected shells of No. 459 at Monterey-, which at 

 that time I called Nacelki im^tabilis^ but these half and half varie- 

 ties did not appear at that time. After a little reflection on the 

 matter, I began to suspect the true condition of the subject and 

 became so much interested in it, that I concluded to go to Mon- 

 terey again and try to work it up, and I think I have done so. It 

 is simply a question of station. 



" When the 3'Oung of A. pel I a stations itself on the kelp {Fhyl- 

 lospora 3Ienziesii^ ^g-\ it assumes the aspect of Nacella, and as 

 long as it remains on the kelp it does not change its color in the 

 least, and only varies its form to suit the shape of the stems of 

 the kelp to which it attaches itself. But when from an}^ cause it 

 leaves the kelp and takes to the rocks, it seems to begin imme- 

 diately to -paint np and ornament itself after tlie fashion of the 

 specimens I have sent yon. 



" When it remains on the kelp a long time and completes its 

 growth, we then have Nacella instabilis, and if living in an exposed 

 position its apex becomes worn, the sculpture faint, etc. When 

 the 3'oung station themselves on the rocks they do not assume the 

 Nacella aspect at all, but commence immediately to adorn them- 

 selves in gay and beautiful eoloi'S as you will see by the fine series 

 of No. 457. Now for the facts and reasons why 1 came to this 

 conclusion. I collected about 200 living specimens on the kelp 

 in all stages of growth, and out of the 200 I found but two speci- 

 mens that varied their color at all ; one was a very young and 

 small s[)ecimen, with a few light dots on or near the apex, and the 

 other was a large specimen with a tesselateil border on the inside, 

 I also collected about 200 on the rocks, and ever}^ one was more 

 or less variegated with eitlier the square dots or alternate rays of 

 white and black, while those that had evidently been on the kelp 

 had their tesselated borders well advanced. We must also take 

 into consideration the fact that Monterey is the most southern 

 point at wliich Nacella instabilis has been found, and it is quoted 

 by Dnll as rare there. The water of the bay where these half and 

 half No. 458 are found, and also others, is comparatively smooth 

 to Avhat it is on the outside where the typical Nacella is found, 

 which will account for the preservation of the apex and sculpture 

 and may have something to do with the form, and undoubtedly is 

 the cause why it is rare at Monterey. 



"To show the effe.ct of station, with probably other causes, I 

 send you a full series of a very pretty var. of A. pelta from Olym- 

 pia, Puget Sound, W. T., 461 to 46'(i inclusive, that I collected 

 last summer. Station, on Mytilus edulis. 



" This very pretty var has almost the exact form and looks like 

 a hnge Nacella p)eltoideii^ Dall, and is a very interesting addition 

 to our lim[>ets." 



