SCIENCE-GOSSIP. 



163 





seem to be taken parallel with the suture. 

 The divergence between the results obtained by the 

 two methods of measurement of diameter may be 

 illustrated by the following example. A series of 

 eighteen specimens of Palttdina vivipara from the 

 Hereford and Gloucester Canal, near Hereford, gave 

 alt. (measured parallel with axis), max. 39 S mm., 

 min. 33 o, mean 34 9 , diam. (perpendicular to axis), 

 max. 27 S, min. 22-7, mean 24 4 ; and diam. (parallel 

 with suture), max. 29 1, min. 25S, mean 273 It 

 is further to be noticed that the two methods of 

 measurement do not agree with each other as to 

 individual specimens. Thus, if we arrange the 

 series above in the order of descending magnitude 

 of diameters perpendicular to axis, and represent the 

 specimens by the numbers 1 to iS, the corresponding 

 numbers of the series similarly arranged according 

 10 the other diameter are: 1. 2, S, 4, 4, 2, S, 6, 13, 

 10, ii, 16, 15, 11. 14. i(3, iS, 17. Part of this is no 

 doubt due to errors in measuring, but part also to 

 a further variation in the shape of the shell, which 

 I hope to deal with later in this paper. 



In measuring bivalves there is : (1) the length, 

 measured from the extreme anterior to the extreme 

 posterior points ; the results are often slightly 

 inaccurate owing to the presence, absence, or 

 varying flexibility of the fringe. (2) The breadth ; 

 the position of this line is rather difficult to define, 

 but not hard to determine in practice. It may, 

 perhaps, best be said to be the greatest distance 

 perpendicular to the lowest part of the lines of 

 growth (i t. the part furthest from the umbones) ; 

 it may be, but not necessarily, perpendicular to 

 the length of line, and one end may or may not 

 coincide with the umbones, more often, perhaps, 

 meeting the ligament at the hinge. (3) The thick- 

 ness is measured with the valves closed tightly 

 together. 



Wcstcrlund (') gives very elaborate instructions 

 (after K J Bourguignat) relative to the measure- 

 ments of bivalves. The process is, however, 

 needlessly complicated, enlailing eleven separate 

 measurement.. It is only useful as a very 

 imperfect substitute for figures, which are really 

 almost mure necessary in Anodonta, Unto, etc., 

 than anywhere else, where the concavity of the 

 lower margin, and especially the form of the 

 posterior end of the shell— two striking and 

 important characteristics -cannot onfortunati 1/ 

 be satisfactorily exhibited in measurer: 



l-nknowin; ngth and breadth 



of a shell, which indie v ■:, it is also useful 



to know the relation of the rements to 



one another, which partly bape 



by calculating tlv 



irmer luting perhaps the 



!-,r ihc flatter, tl t the 



more produced species. By this means it is 

 possible to express the two great deviations of 

 shape in Gastropoda, conoidity and globosity, in 

 terms of numerals. Further, the varieties conoidca, 

 globosd, etc., may be thus more closely defined, if it 

 is deemed more convenient to call them by names 

 than by numbers. In the case of a shell such as 

 Tachea ncmoralis, for example, it is found that in a 

 series of such fractions, worked out from measure- 

 ments of altitude and diameter, there are a good 

 many near one another. This is the normal 

 degree of globosity or conoidity for the particular 



series. In others the fraction '?""''" gradually 



altitude ° J 



rises to a maximum ; these are the globose ones ; 



while at the other end there is a corresponding fall 



which represents the conoid specimens. I hope 



later to discuss this point more fully, but at present 



I will only give the following examples : 



Pat ula rotundata, Here- 



DIAM. 



ALT. 



DtAM. 

 ALT. 



fordshire, Silurian.. 



6.0 mm. 



2-5 



mm. 240 



(Above 2 = flatfish.) 



63 



2-8 



2'2S 





5 5 



2-8 



1-96 





62 



3-2 



1-94 





57 ' 



3° 



1 -go 





ALT. 



DIAM. 





C. aspersus, Hereford 



326 



36-8 



1129 



(Anything less than 



2S"4 



3I-S 



I-I20 



1 is quite conoid.) 



311 



33'9 



I OgO 





311 



305 



gSi 



Ch. lapicida, Hereford- 









shire 



90 



i6'7 



186 



(Less than 2 = high- 



0-2 



174 



1 89 



spired.) 



8.4 



168 



200 





7-9 



176 



223 





6-9 



160 



232 



Bid. obscurus, Hereford- 







ALT. 

 DtAM. 



shire . . ... 



«4 



35 



24O 





85 



3-6 



236 





S 2 



3 5 



234 





77 



33 



'■U 





8'3 



V6 



2305 





7-8 



! 1 



229 



imiiicum, llereford- 



1 ." • ' .. 



WIDE. 



LENGTH . 

 BREADTH, 



hiie 



6-4 



4 7 



1-36 





57 



4'2 



1 |6 





74 



i-e 



i'32 





67 



5 2 



1 'i 





80 



6 1 



125 



1 



oiitfu ucd 1 







Si.rii Fungi i am 1 nil'' ting spei imens and 



.1.11 ...1 ill'- Mj "in')' etc ■, "i slime fungi, and 



thinl< the 1 1 plan will be to ask foi help through 



• of Go N-. Will any but. mi .1 



kindly assist me' W II Pepworth, The Avenue, 

 ■..Citcihirc, Seplembir 2yth, 1897, 



