DISCOVERY OF HYGROMIA UMBROSA Partsch 

 IN ENGLAND. 



Bv JOHN \V. TAYLOR, AI.Sc. 



(Read before ihe Society, Dec. 8th, 1915). 



Mr. J- C. Dacie, a devoted student of the Littorinidce, and a valued 

 member of our Society, was so fortunate as to find living specimens 

 of Hygroinia unibrosa at Margate, during September of last year. 

 They were living in association with H. stn'olata, H. cantia?ia, and 

 other species, and add another interesting species to our fauna. 



This very distinct species has, however, been previously twice 

 recorded as British by Messrs. Kennard and Woodward (Proc. Malac. 

 Soc, Nov., 1897, p. 243 and figs.; and The South-Eastern Naturalist, 

 1905, pp. 4, 5), both records being based upon four fossil shells of 

 Pleistocene age, found by the distinguished palaeontologist, Mr. W. J. 

 Lewis Abbott, in the Ightham fissure, Kent ; but these specimens 

 were afterwards considered to be modified ffygro/nia sfrio/ata, and 

 the records have now been withdrawn, so that Mr. Dacie's find is 

 really its first undoubted occurrence in this country. 



Mr. Dacie was at first inclined to regard the shells as a somewhat 

 peculiar form of H. sl/iolata, and as such exhibited his specimens 

 at a meeting of the London Branch of the Conchological Society ; 

 but at this meeting the members present were so impressed with the 

 peculiarities presented by the shells that Mr. Dacie was urged to 

 send them to myself for examination. 



Hygroinia umbrosa may be easily distinguished by many striking 

 testaceological characters from H. siriolata, with which the Margate 

 shells were at first confused. Amongst its distinguishing features 

 may be mentioned the distinctly granulate surface of the shell, as 

 opposed to the simple striation of Hygroinia striolata ; the much 

 greater tenuity and transparency of the shell substance, and its 

 usually peculiar greenish tinge ; the excessively oblique and some- 

 . what expanded aperture ; the closely-approximating margins, with 

 little or no trace of the submarginal rib, so perceptible in H. striolata ; 

 the widely expanded umbilicus, etc. 



Little is known of the internal organization of H. unibrosa, but it 

 is credited with possessing only one love-dart, which is described as 

 smooth and conical, and it is thus removed from intimate alliance 

 with H striolata, which always possesses a pair of these weapons, 

 and, in addition, an extra pair of accessory glandular sacs. 



The distribution of this species is another illustration of the expul- 

 sion of a subdominant or weaker species from the active evolutionary 



