194 



SCIENCE-GOSSIP. 



COLORATION AND VARIATION OF BRITISH 



EXTRA-MARINE MOLLUSCA. 



By Arthur E. Boycott. 



(Continued from page 1G3J 



T HAVE omitted to mention that in measuring 

 very flat forms, such as Planorbis, two altitudes 

 should be taken : one in the centre, down the 

 columella (this is conveniently done with the 

 curved limbs of the Boley gauge) ; the other, the 

 greatest thickness of the last whorl (with the flat 

 limbs). The former might be called the " apex," 

 the latter the "total height." The same thing 

 holds good for such involute forms as Limnaca 

 peregra involuta and some forms of auricitlaria. 



For the study of variations in colour no satis- 

 factory plan seems to have been devised. Descrip- 

 tions of colours in words- are all very well, but 

 it is difficult to define the exact tints conveyed 

 by such terms as " reddish," " dark olive," " pink- 

 ish," or even "brown" or "red." A universal 

 colour scale would be very convenient if it included 

 the tints of shells. Roughly and generally, of course, 

 there is little ambiguity. 



Weight is easily determined by a balance ( 1 ). 

 It is well to weigh to milligrammes (0001 grms), 

 but unnecessary to go beyond centigrammes (o - oi), 

 in the case of the larger species at any rate. The 

 difficulty of getting the shells quite clean, and at 

 the same time quite unbroken, renders too refined 

 weighings useless. Indeed, it is very doubtful 

 whether in the case of Tachea, for instance, the 

 weighings can, as a rule, really be trusted below 

 centigrammes. A very small bit of " body " left 

 inside makes much difference. The degree of dry- 

 ness of the shell of course introduces a source of 

 error ; it is most convenient not to weigh any 

 which have not been cleaned out at least some 

 months. After this period the weight probably 

 keeps fairly constant, if one may judge from a 

 short experience of two years or so. 



After these preliminary observations on method, 

 it will be well to consider what is known about the 

 normal form and coloration of our British extra- 

 marine mollusca, before going on to consider the 

 many variations in these respects which are found. 



The normal gastropod shell-form is perhaps a 

 rather short, wide cone, as found in Helix. The 

 original form was presumably something similar in 

 external shape, but simple, without any convolutions, 

 such, in fact, as is found in Patella, Ancylus and 

 Dentaliiim, e.g., at the present time. The heliciform, 

 coiled, spiral shell can have arisen from an elongated 



(') F. E. Becker and Co., Hatton Garden, London, E.C., 

 supply a simple, cheap form admirably adapted for the 

 purpose. 



cone like Theca (Cambrian) or Dentalium (1) by 

 coiling in one plane, eventually giving rise to a 

 Planorbis shape, which, by an elevation of the 

 central apex from the plane of the shell and a 

 descent of the last whorl, gave the Helix form (') ; 

 or (2) by coiling in a spiral form, which by 

 compression produced a Helix, and, by further 

 compression, a Planorbis shape. The former view 

 has been advocated by, e.g., E. W. W. Bowell (-), 

 the original incurving of the cylindrical, tapering, 

 simplecone being due to " mechanical convenience " ; 

 it is supported by the fact that instances where a 

 long cone is curved in one plane at one end are 

 found in recent and fossil forms. Caecum trachea, 

 for instance, when young exhibits just the hypo- 

 thetical bending in at one end, while the rest remains 

 a straight tube. Crioceras, Ancyloceras and Hamites 

 (from the Gault) exhibit similar forms with varying 

 details among the Cephalopoda, which, it may be 

 noted, also show Planorbis and Helix forms of shell. 

 On the other hand, the second view is supported 

 by such palaeontological specimens as Heiicoceras 

 (Gault), or the recent Cylindrella, and by the scalari- 

 form variation which now so often occurs (this 

 may be due, of course, to pathological causes), and 

 perhaps further by the spiral form of the embryonal 

 shell in such highly modified forms as Ancylus, 

 Fissurella and Limax. This, again, may be but an 

 indication of the spiral form from which they have 

 doubtless been immediately derived. It appears 

 that the former view is generally held, and Tryon ( s ) 

 seems to agree with it. 



In whatever way the spiral shell and the torsion 

 of the visceral hump has been produced, however, 

 it seems that the somewhat globose shape, as re- 

 presented in Tachea, combines great strength with 

 a comparatively thin shell. Shells of this shape 

 (Cryptomphalus, Helix (restricted), Tachea, Xerophila, 

 part) often live in open places, where their chance 

 of receiving various knocks is a very good one, 

 and it requires some pressure to crush a well- 

 grown Tachea. Thin elongated shells are often 

 of use to snails in making their way about in 

 nooks and crannies, among stones, under bark on 

 trees, etc. This is the case with Clausiiia, Balia, etc. 

 A great reduction in the size of the shell enables 



(>) Some specimens of PI. comcus (Oxford) look as if they 

 were trying to do this now. 



{-) Science-Gossip, June, 1892. 



( 3 ) Struct, and Syst. Conch. (18S2), i„ p. 37- 



