HARDWICKE'S SCIENCE-GOSSIP. 



235 



have been specially got up for students of "Sack 

 Thorne" and other botanical manuals on vegetable 

 physiology. Mr. White's list includes no fewer than 

 1 So such illustrative slides. Whilst speaking of such 

 aids to biological students, we would draw attention 

 to the preparations and specimens of Mr. W. K. 

 Mann, of Bristol, whose catalogues collectors would 

 do well to obtain and study. All instruments for 

 egg collecting and preparation are offered by him, as 

 well as collections of all sorts of insects, shells, birds' 

 eggs, skins, &c. — From Messrs. Bolton, of Birming- 

 ham, may be procured mounted specimens of a vast 

 number of alga.', infusoria, rotifera, polyzoa, &c, for 

 students' uses. 



The Royal Microscopical Society. — In addi- 

 tion to the usual welcome summary of current re- 

 searches relating to zoology and botany, the August 

 part of the journal of the above society contains a 

 lengthy and highly elaborate paper (well illustrated) 

 on "Additions to the Knowledge of the Carboniferous 

 Foraminifera," by the Rev. Walter Howchin, F.G.S. 



Measurements by Camera Lucida. — The books 

 that R. W. has are quite correct when they state 

 that the microscope when 'horizontal, that is, the 

 •eye looking through the camera lucida, must be 

 10" from the table, or, rather, the paper on which 

 the drawing or measurement is to be made. R. W. 

 is quite wrong in his surmise, that the distance 

 from the paper should vary with the distance between 

 the object and the eye-piece. When we say a 

 •drawing magnifies so much, we mean that it 

 magnifies the appearance of an object at a certain 

 distance so much, and this distance for microscopic 

 purposes is by general consent 10". A man at a 

 thousand yards appears a very different size to what 

 he does at a hundred yards. If we have a drawing 

 which states that a man is magnified so much, we 

 must know at what distance the man is supposed to 

 be. If we place in the horizontal microscope a scale 

 of hundredths of an inch, and looking vertically 

 down through the camera lucida on a piece of 

 paper at 10" from the eye, see that three of the 

 divisions equal one inch, we then know that the 

 picture we see or draw magnifies the object J4|9 = 33 

 times nearly, supposing the object to be at 10". — ■ 

 F. Le Grice, Colonel R. A., Shorncliffe. 



ZOOLOGY. 



Anodonta cygnea, Linne. — In the September 

 number of Science-Gossip Dr. Williams states 

 concerning the form Anodonta anatina, Linne, that, 

 " Anatomical differences apart, there is enough (he 

 docs not say what) in the shell to warrant us in still 

 holding on to it as a good species." He has just 

 before said that Dr. Henry Woodward's statement 

 to the contrary was " uncanonical." If reference be 



made to the principal authorities on this subject, it 

 will be found that they by no means endorse the 

 opinion held by Dr. Williams. Messrs. Forbes and 

 Hanley,* who were naturalists in the highest sense of 

 the word, mention the name " anatina " as belonging 

 to a variation in the form of the shell of Anodonta 

 cygnea. Isaac Lea,f who made a life-long study of 

 the unionidre, and added more species to this family 

 than any other man, says, when speaking of the 

 point in question : " I have after a good deal of 

 consideration and examination of my specimens, 

 and the figures in numerous works describing the 

 unionidce, satisfied myself that An. cygnea and An. 

 anatina are not specifically distinct." Dr. Paul Fischer 

 again, in his " Manuel de Conchyliologie " (p. 197), 

 has quoted, without amendment, from Dr. Gwyn 

 Jeffery's book, % the number of species in the genera 

 of British land and freshwater mollusca, two species 

 of anodonta being enumerated, but when speaking of 

 the distribution of this genus, Dr. Fischer says there 

 is but " one European species."§ Turton and Gray 

 held the same opinion, the latter agreeing with 

 Forbes and Hanley || that not even good varieties 

 could be made from the variations of the freshwater 

 mussel. Surely in the face of authorities such as 

 these it is, to say the least, premature, to raise the 

 charge of " uncanonical," for the onus probandi rests 

 with those who hold the contrary opinion. The 

 whole difference between the shells of the " two 

 types " so-called, can be summed up in this. The 

 ventral margin is approximately parallel to the hinge 

 line in the typical cygnea, while in the other form 

 the hinge line, and the approximately straight line of 

 the ventral margin, if produced will meet on the 

 anterior side of the shell. On turning to the 

 descriptions of these forms in Dr. Williams's 

 Handbook, no anatomical differences are to be 

 found noted, though the author of it in the article 

 mentioned above seems to consider that they exist. 

 The shell of cygnea is described as " oval," that of 

 " anatina" as " elliptical- oval," one fails to see any 

 essential difference. Again the cygnea shell is called 

 "dull green," that of the other form, "dark-olive- 

 coloured ; " now the largest specimen of anodonta 

 in my collection (six inches from anterior to 

 posterior margin) which undoubtedly is cygnea is 

 dark-olive-coloured, while another specimen which is 

 a very good example of the variation is dull green. 

 Any one who has collected and examined fresh-water 

 mussels knows that there is every conceivable interme- 

 diate form between the typical cygnea and the so- 

 called species anatina, and that the shells vary in many 

 ways besides, according to their environment. A 

 very good way to obtain evidence bearing on the 



* " British Conchology," vol. ii. p. IS9- 

 t " Synopsis of the Family Unionidse," p. 

 J " British Conchology." 

 5 " Manuel de Conchyliologie," p. 1002. 

 II " British Conchology," vol. ii. p. 156. 



76. 



