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BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



when broken across or viewed from the base; by the poreless, cal- 

 careous basis, the longitudinally striate scutum, and the narrow spur 

 of the tergum. It is closely related to the north Pacific B. evermanni, 

 but differs by the distinct longitudinal striation of the scutum, 

 which invariably, in a vast number of specimens I have particularly 

 examined, has a thin, well-raised articular ridge, culminating below 

 in an obtuse but very distinct angle, below which it tapers downward. 

 In B. evermanni the exterior of the scutum is flat, without longitudi- 

 nal striation, and the articular ridge is extremely low, rounded, and 

 not angular. The spur of the tergum is narrower in B. evermanni. 

 There are also important differences in the compartments. The 

 wall is thinner than in B. evermanni, its inner surface more ribbed; 

 the basal edge has fewer septa, which are subequal, more spaced and 

 different in shape. 



The size and shape are variable. The following measurements of 

 American specimens give the maximum and the usual size attained 

 in our waters. 



Darwin mentions specimens from Scarborough 2 inches in diam- 

 eter, If inches high, and l.G inches diameter, 3 inches high. The 

 usual dimensions are therefore about the same in northern European 

 and American waters. Pleistocene specimens from Uddevalla, 

 Sweden, are said to be nearly 4 inches long, therefore about equal 

 to the largest American individuals. 



They grow upon any available object, usually on shells and upon one 

 another. The commonest situs in our waters is on Chrysodomus 

 decemcostatus, Buccinum undatum, Pecten magellanicus, Modiolus 

 modiolus. More rarely I have seen them on Terebratulina septentri- 

 onalis, the carapax of Ilyas, and on stones. 



By far the greatest number of specimens in the museum are from 

 between 30 and 55 fathoms. It is abundant on the fishing banks, and 

 is frequently brought up on cod lines. Like B. balanws, it is wanting 

 on the Newfoundland Bank, and none have been obtained in a 

 greater depth than 1G7 fathoms. The recorded temperatures are from 

 about 39 to 58. 



