By William Healey Dall. 



Curator of Mollushs, United States National Iluseum. 



NOTES ON THE SPECIES OF THE MOLLUSCAN SUBGENUS 

 NUCELLA INHABITING THE NORTHWEST COAST OF 



(AMERICA AND ADJACENT REGIONS. 



but not the Purpura of the classical writers nor of Martyn in 1784, 

 are so far as known confined to the Northern Hemisphere, and 

 flourish especially in its cooler waters, some species extending their 

 range nearly to the Arctic Circle, The type of the subgenus is the 

 sohtary North Atlantic species N. lapillus of Linnaeus, the genus 

 Thais Bolten, covering both the northern and tropical groups. The 

 synonymy has been exhaustively treated in another publication.^ 

 The history of N. lapillus has been worked up by the Rev. Dr. A. H. 

 Cooke in a recent paper ^ and he included a notice of the Pacific 

 species, the synonymy of which had been previously considered by 

 Vanatta.^ 



The very rich series of these shells contained in the United States 

 National Museum and the doubts expressed as to the relations of 

 several of the nominal species suggested that a review of the North 

 Pacific forms might properly supplement Doctor Cooke's admirable 

 paper on the Atlantic species. 



The nucleus of the northern species of Nucella is smooth, white, 

 sUghtly gibbous, with an apical dimple and one and a half 

 whorls. No essential variation from this type has been noted in 

 any of the species. In adults it has generally been lost through 

 attrition. The extreme tip is bulbous and relatively large. The 

 transition from the nuclear to the adolescent stage is abrupt, the 

 newer surfaces taking on at once, above the suture, two or three 

 strong spiral ridges. 



The nucleus of the tropical species is different. It is very difficult 

 to obtain specimens in which it is intact, even in the very young. 



1 U. S. Geological Survey, Professional Paper No. 59, Washington, 1909, pp. 46-51. 



2 Proceedings of the Malacozoological Society of London, vol. 11, 1915, pp. 192-209. 

 « The Nautilus, vol. 24, 1910, pp. 37-38. 



Proceedings U. S. National Museum, Vol. 49-No. 2124. 



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