32 Mr. W. Clark on the Lacunae. 



verge is a long, simple, arcuated, flattish, pale yellow process, re- 

 gularly tapering from its insertion under the right tentaculura 

 to a fine point. The operculum is pale horn-colour, subelliptic ; 

 nucleus excentric, sublateral, abutting on the columella, with 2^ 

 volutions, the last of which rapidly unites with the margin. We 

 repeat the remark on the internal organs of the Littoriiia litto- 

 ralis, in comparison with this species, that they are identical. 



I conclude this portion of my paper by a few comparative ob- 

 servations. Malacologists will observe that in the two species 

 the internal and external organs are essentially the same ; the 

 only variations are, that in Littorina pallidula, the caudal fila- 

 ments, the expansions of the operculigerous lobe and the form 

 of the organe generateur, are not precisely similar to the parts 

 in jL. littoralis ; the former are very variable specialties and can- 

 not be depended on : ex. gr. the Lacuna puteolus of authors is 

 without caudal processes, or at least they are obsolete ; the same 

 observation may be made of the Rissoce, no two animals of the 

 same species being similar in respect of caudal accessories, and 

 many are without them. The male organs of the two undoubt- 

 edly vary in structure ; the groove or lacuna in the pillar, from 

 whence the generic title, is a mere modification of the umbilicus, 

 which, from its variableness, is not of specific value, and is quite 

 misapplied to generic determination. All the variations we have 

 mentioned are specialties of slight consideration, which cannot 

 be put in competition with the essential generic characters of 

 the foot, and its singular plan of locomotion, which stamp the 

 so-called Lacunce of authors as genuine members of the genus 

 Littorina. 



Addenda to the memoir on the Littorinidae, page 352 of the May 

 'Annals,' 1850. 



Littorina littorea. 



The pectinations of the branchial plume of nearly all the Lit- 

 torina, instead of being 16-20, vary from 45-60 ; they are pale 

 brown, long, slender and close-set ; an examination under the 

 microscope sanctions this correction. Having repeated, this 

 20th of May, a series of fresh observations on some hundreds of 

 the Littorina rudis in comparison with the L. tenebrosa and L, 

 jugosa of authors, I am authorized to say, that the identity in 

 figure and action of their internal and external organs has further 

 satisfied me of the propriety of regarding them as varieties of the 

 L. rudis; the only differences are colour and size, resulting from 

 the various complicated incidents attached to habitat. I omitted 

 to observe that the Littorince are destitute of head and neck 



