l62 



SCIENCE-GOSSIP. 



me every satisfaction, is a small clockmaker's 

 gauge (measuring up to 10 cm.) made by Boley, 

 of Esslingen ('). It is a slide-gauge, and reads with 

 a vernier to 01 mm., which is sufficiently accurate 

 for ordinary purposes. It is always well to err 

 on the side of excessive minuteness, as anyone 

 who has tried extensively knows that the personal 

 equation and the equation of the moment may 

 mean a more or less a considerable percentage 

 variation in the results on the same shell measured 

 by different persons or at different times. A gauge 

 such as this, as ordinarily used, may probably be 

 regarded as accurate to o-2 mm. 



Quite small species, such as Vertigo, Cavychium , 

 should perhaps be measured to o or mm. It is 

 practically impossible to do this with a sliding 

 instrument, indeed it is dangerous with any sort of 

 gauge in which the shell must be held fast between 

 two hard points. The simplest way seems to be to 

 measure them under the microscope with a power 

 pf 50 diameters or less. The practical difficulties 

 of successfully applying the slide-gauge to such 

 shells as Vitrea, Balia, etc., are also not inconsider- 

 able. It is easier to measure these on a microscope 

 with a travelling stage fitted with scales and 

 verniers. It need hardly be mentioned that a 

 qualification which any practical measurer for 

 these purposes must possess, is ease and rapidity 

 of working. Life is much too short to make 

 two or three measurements and a weighing on 

 many thousand specimens up to 001 mm. and 

 o'oooi grammes, at least this is so to ordinary 

 mortals. 



The question next arises, in what direction 

 exactly the measurements are to be made. Most 

 books simply say : altitude, so many millimetres, 

 or lines, etc., diameter, so much ; unless they 

 assure us that the species is about as big as a 

 hazel-nut, or some other object of unvarying and 

 immutable magnitude. Is the altitude measured 

 parallel with the axis (columella) of the shell ? Is 

 it the extreme length from the apex to the most 

 produced part of the mouth, or from the apex to 

 the umbilicus, along the axis ? Is the diameter 

 taken parallel with the slant of the whorls, or at 

 right angles to the altitude ? 



C. A. Westerlund f 2 ) distinguishes two measure- 

 ments of height (altitude e.g., in "Helix," Hyalinia), 

 or length (longitudo, e.g., in Buliminus, Clausilia) : 

 (1) altitudo major, from the apex to the lowest 

 part of the mouth : he appears to say that this 

 distance is measured parallel with the axis ; (2) 

 altitudo minor, from the apex to the middle point 

 of the underside of the shell, i.e. the umbilicus. 



(M It may be procured from Messrs. Grimshaw and Baxter, 

 31 and 35, Goswell Road, Clerkenwell, London, E.C., price 

 5s. It has the disadvantage of not being long enough for 

 large bivalves, nor hardly broad enough for such species as 

 H. pomatia ; it takes Cryptomphalus easily enough. 



( 2 ) Fundamenta Malacologica (1892), p. 71. 



The plan which I have provisionally adopted 

 is as follows. 



All helicoid shells have their altitude measured 

 parallel with the columella, or assumed columella, 

 and from the apex to the extreme point of the 

 mouth or point on the last whorl which is most 

 distant from the apexf 1 ). That is to say, the altitude 

 lies along a line drawn from this point parallel with 

 the axis to a point where it meets a line drawn 

 laterally from the apex at right angles to the axis. 

 The diameter is the greatest distance from the 

 peristome to the exposed part of the last whorl. It 

 is not perpendicular to the altitude line, nor parallel 

 with the slant of the whorls, but is intermediate 

 in position in Cryptomphalus or Tachea. In the 

 flatter snails it is almost or quite perpendicular to 

 the altitude line. The chief point to be careful 

 about in measuring such shells as Tachea and the 

 other Helix genera, is to have the columella parallel 

 with the long limb of the gauge. It is of necessity 

 a point in which one cannot altogether exclude 

 variation dependent on the time or individual. 



In passing, I may mention that there is another 

 method of measuring C. aspersus, which gives rather 

 interesting results. The altitude is measured 

 diagonally from the apex to the furthest point on 

 the peristome, and the diameter is the distance 

 from the peristome to the exposed part of the last 

 whorl, when the shell is resting naturally with its 

 aperture downwards. The measurements are more 

 adapted for use with callipers (altitude), and direct 

 reference to the scale (diameter). If now the fraction 



'- is taken, those shells in which it is less 



altitude 



than one may be called conoid, those in which it 



is more than one, globose. 



In shells like Paludina, Cydostoma, Buliminus, 

 Clausilia, etc., the altitude is again parallel with, 

 and almost along, the columella ; and the diameter 

 is the greatest width perpendicular to this. All 

 other univalve British forms fall naturally under 

 one of these two heads. It is important to 

 remember that the "real" altitude of Planorbis 

 must be measured in the centre along the columella : 

 this is done with the curved limbs of the Boley 

 gauge. 



These two methods of diameter measurement 

 are not, I think, particularly disadvantageous, as 

 the object is not so much to make comparisons 

 between the sizes of different species as between 

 individuals of the same species, and to have a 

 method which gives accurate and regular results, 

 while it is easy and rapid of execution. 



Westerlund distinguishes latitudo major, which 

 is the whole breadth of the shell measured to the 

 outer edge of the peristome, from latitudo minor, 

 which is measured at right angles to this. Both 



(') In Tachea it is sometimes a point on the peristome, 

 sometimes on the last whorl. 



