Retrospective Criticism, SOS 



the discovery I was fortunate enough to make of the metamorphosis in the 

 Crustacea : thus you state that I " had the satisfaction of witnessing the 

 metamorphosis j^rs^ described by the Dutch naturalist Slabber;" now if you 

 will take the trouble of reperusing my first Memoir, p. 8, and of inspecting 

 Slabber's two figures copied into my first plate (fig. 1. a and 1. b,) you will 

 readily perceive that, allowing Slabber to have seen what he describes, it is, 

 at most, a change from one form of a natatory crustaceous animal into 

 another natatory form wholly unknown, except to Slabber himself, and is 

 hence a change which could never authorise an idea of the Dec^poda 

 undergoing metamorphosis. As you may observe, I doubt altogether that 

 Slabber witnessed my change, and shall shortly show what his fig. 1. b really 

 is, this constituting one of the many discoveries I have more lately made. 

 I should probably have published this in my next Number, but that I have 

 a still more interesting memoir on the Cirripedes prepared for it, which 

 will develope mysteries still more unexpected and important than the dis- 

 covery of the metamorphosis in the Decapodous Macroura. — J. V. Thom- 

 son, ilfrt^ 21. 1829. 



A singular Nidus. — I possess one of the same singular species of nidus 

 as that mentioned by your correspondent (p. 104.). I, however, doubt its 

 belonging to an aquatic insect, as mine was found by a friend (a botanist) in 

 a hedge in a lane, and is attached to a small twig of the blackthorn. I 

 have never met with it myself, although I am in the constant habit of 

 exploring the countrj', at every season of the year, for insects and shells. — 

 Charles Blomen. Teignmouth, Devonshi7'e, Aprils 1829. 



Bulimiis acutus and Helix virgata 

 (fig. 40. p. 150.) were not distinguished 

 in a part of our impression; the cut is 

 therefore repeated (^g.S 7.); the snail ^^-z 

 (Helix virgata) being marked^, and the "^^^^^ 

 Bulimus a. '^C 



British Land and Fresh-water Shells. — 

 Sir, I will thank you to make the following alterations in my paper on 

 British Shells (Vol. I. p. 424.) : — 



After the observations on Lymnae^a fragilis add, " Is this shell the imma- 

 ture L. stagnalis, or a variety of it? I have specimens of a Lymnae'a sent 

 to me as the L. stagnalis, agreeing with Montagu's figure 7. plate 16., which 

 is referred to as a representation of L. fragilis. I find little difference in 

 the descriptions and figures, except in size." 



After those on Helix fusca add, " The Helix subruf^scens of Fleming is 

 only the young of this species, according to the observations of W. Bean, 

 Esq., of Scarborough." 



After those on H. carthusianella add, " The specimens labelled H. car- 

 thusianella in the Manchester Museum, however, are much less than either 

 Draparnaud's figure or our English shell." 



After those on H. ruf^scens add, " Since writing the above, I have 

 observed in the Manchester Museum this shell designated as the Helix 

 cornea. I believe the label is in the handwriting of W. Svyainson, Esq., 

 but may have possibly been subsequently misplaced ; for certainly our Eng- 

 lish shell differs very materially fiom the H. cornea of Draparnaud, to 

 whose figure Lamarck refers." Yours, &c. — Joseph Kenyon. 



Spinning Slugs (p. 69.)- — It is long since this supposed species of slug has 

 been described. On very many occasions have I observed the descent of 

 the common grey and common green-bellied slug from trees and bushes, but 

 without ever having reason to suspect that there was a distinct species, 

 having the property of spinning a web. The fact is, that any small snail, 

 in good health, can let itself down from almost any height to the ground,, 

 by the tenacity of the slime exuded retromissively from the pores of 

 the belly. The same muscular motion of the belly, exerted on givmg 



X 4 



