138 PROCEEDINGS OF THE NATIONAL MUSEUM vol. i09 



Subclass EUTHYNEURA 



Order Opisthobranchia 



Suborder Tectibranchiata 



Family Acteocinidae 



Genus Retusa Brown, 1827 

 Retusa umbilicata (Montagu, 1803) 



Plate 4, figure 6 



Bulla umbilicata Montagu, 1803, ed. 1, vol. 1, p. 222. 



Cylichna nilidula Lov4n, 1846, p. 142. 



Cylichna strigella Lovdn, 1846, p. 142.— Tryon and Pilsbry, 1893, vol. 15, p. 210, 



pi. 27, figs. 7, 8 ; pi. 29, figs. 11-14. 

 Utriculus nitidulus G. Sars, 1878, p. 286, pi. 17, fig. 13; pi. 26, fig. 3. 

 Utriculus umbilicatus G. Sars, 1878, p. 286, pi. 17, fig. 14. 

 Retusa umbilicata Lemche, 1948, pt. 1, p. 81, fig. 45; pt. 2, p. 81, figs. 59-61. 



A single dead shell of this species was taken on Sept. 6, 1949, at a 

 depth of 477 feet. The shell is 2.8 mm. in height by 1.5 mm. in width. 



Other material examined: Approximately 200 specimens from 

 the Shetlands, Scotland, Ireland, Sweden, Norway, the Bay of Biscay 

 (1 specimen) and the Gulf of Naples (1). 



Discussion: Lemche (1948) discusses and figures many of the varia- 

 tions found in this species. Among the specimens I examined there 

 were some with a large umbilicus, others with a small umbilicus, and 

 still others with no umbilical perforation; in some there were distinct 

 axial striae and in others such striae were lacldng; in some the apical 

 perforation was distinctly visible, in others it was feeble, and in still 

 others it was lacldng. One finds all possible combinations of these 

 various characters; for example, one shell may have striae and an 

 umbilicus, another may have striae and no umbilicus and yet another 

 may have neither. 



In the specimen from Point Barrow (pi. 4, fig. 6) the apical perfora- 

 tion could be considered lacking or very feeble, the umbilical perfora- 

 tion is lacking, and longitudinal striation m&y be said to be lacking, 

 for even under high magnification there is such a famt indication of it 

 that one could scarcely term it striation. It is similar to figure 13 of 

 G. Sars (1878) and figure 11 of Tryon and Pilsbry (1893). In the 

 majority of other specimens that I examined the axial striation was 

 lacking. 



Distribution: Point Barrow, Alaska; the east coast of North 

 America; Norway and Sweden, the British Isles, the Shetlands, the 

 Bay of Biscay, and the Gulf of Naples. 



