300 



BULLETIN" 93, UNITED STATES NATIONAL MUSEUM. 



and tapering below. The articular ridge of the tergum is very small, 

 and the articular furro^v is narrow. The bilateral asymmetry of the 

 opercular valves, though not conspicuous, is noticeable in some of this 

 lot (fig. 83). I have often observed such asymmetry in this and other 

 species of Chthmnalus, but never in any other genus of Balanidce or 

 Chthwrnalkla. The specimen I dissected from Ocean City differs a 

 little from that taken at station 2004 in the second cirri, as I have 

 noted above, but not in tiny other character of the mouth or cirri. 



Excepting Darwin's Charleston record for fragllis, the only pub- 

 lished notice of Chthamtdus on our Atlantic coast is contained in an 



article by Dr. F. 

 B. Sumner (1909). 

 Specimens identi- 

 fied by Doctor Sum- 

 ner as C. stellatus 

 var. fragilis were 

 found 



to occur in consider- 

 able numbers on Pen- 

 zance Point, along the 

 shore of Woods Hole 

 Passage. Further 

 search has revealed 

 its presence on the 

 piles of piers at Woods 

 Hole, New Bedford, 

 and Vineyard Haven, 

 and on rocks at Nobska 

 Point, Nonamesset 

 Island, and the shore 

 of Buzzards Bay near 

 Woods Hole. It is 

 probable, indeed, that its local distribution is very general. At the last-named 

 point this species seems to be particularly abundant. It extends consider- 

 ably higher up on the bowlders than does Balanus halanoides, with which 

 however, it is associated at a lower level. It thus occurs at points which 

 must be uncovered by the tide for the greater part of the time. In local waters, 

 so far as I have seen, Chthamalus never grows in such dense clusters as does 

 Balanus halanoides, and indeed it appears unable to compete very successfully 

 with the latter in its proper zone. Like its associate, it is a strictly littoral 

 form and probably does not extend below tidal limits. 



The local examples, in large part at least, seem to belong to the variety 

 fragilis of Darwin, as did the specimens received by the latter author from 

 " Charlestown " (^Charleston ?). A characteristic of this variety is the smooth, 

 delicate appearance of the valves, referred to above as distinguishing local 

 specimens. At Woods Hole I have fotind few having the rugosity, the weathered 

 aspect, or even the whiteness of Balanus halanoides. 



Our local representatives of the species are so much darker in color and 

 so much smoother in appearance than the associated Balanus as to be plainly 



Fig. 83. — Chthamalus fuaoiliSj Ocean City, N. .T. Oper- 

 culatt valves of one individu.^l, showing slight bi- 

 lateral asymmetry. 



