THE SESSILE BARNACLES. 171 



stomachs of cod. On Georges Bank it is common on Pecten magel- 

 lanicus and Bucclnum undatum; and in the shallow waters of Vine- 

 yard Sound it occurs commonly on Balanus hameri, Cyprina islandica 

 Fulgur, Polinices, Pecten, Cytherea, Mytilus edulis, on bark, etc., 

 Darwin notes that B. crenatus sometimes incrusts vessels, and that 

 it never occurs above low-water mark, while B. Icdanoides is strict!} 7 

 limited to the intertidal zone, and does not extend into the zone of 

 B. crenatus. Sometimes the two are found growing together where 

 their zones meet. 



Most of the American records are from low tide to 50 fathoms, 

 but in some cases it has been taken in deeper water, up to 98 fathoms. 

 The absence of B. crenatus from the great number of dredgings off 

 our coast in 100 fathoms and over is good evidence that its vertical 

 range is limited rather sharply. 



The temperature records show a wide range from 30 F., on the 

 Grand Banks, to 67 F. in Vineyard Sound, 9 fathoms, in July, but 

 no doubt individuals from low-tide stations experience greater ex- 

 tremes of temperature in the course of a year. 



In the estuary of the Exe Elver, Channel coast of Devon, B. 

 crenatus lives where there is some admixture of fresh water, below 

 low tide, according to the labels in the Jeffreys collection. Speci- 

 mens on potsherds (pi. 39, fig. 1) are patelliform, also those on 

 Mytilus, where it occurs with similarly shaped B. balanoides. On 

 Pecten, doubtless in deeper water, the usual steeply conic form is 

 found. 



In Alaska and on the Pacific coast B. crenatvs seems to be re- 

 stricted to much shallower water than in the Atlantic. Nearly all 

 the records are from collections made on shore or in shallow water, 

 while it is conspicuously absent from most of the Albatross hauls 

 in deeper water. 



Pacific and Bering Sea forms. 



The occidental forms of crenatus do not differ materially from 

 those of the North Atlantic. There is the same range in size. The 

 smooth form (pi. 41, figs. 2, 2, 26, Tacoma, Washington, Cat. No. 

 48033) and the rudely plicate form (pi. 41, figs. 3, 3, Bering Sea, 

 and pi. 41, fig. 5, Alaska, Corwin, Cat. No. 48035), with various in- 

 termediate stages are of common occurrence. The opercular valves 

 resemble those of Newfoundland and Vineyard Sound specimens. 

 The articular ridges of both valves vary in the degree of reflection; 

 that of the tergum sometimes stands vertical to the plane of the 

 valve, or sometimes is reflexed or hooked in varying degree over the 

 articular furrow. The articular ridge of the scutum is usually 

 shorter in Pacific forms. 



A smooth, conic form with rather wide radii was taken by the 

 Albatross in Union Bay, Bayne Sound, British Columbia, on shore, 



