Geological Society. 75 



The genera here described he classifies in three groups, as 

 follows : — 



" I. The first group, including the genus Cardita and the family 

 Veneridit', is distinguished by the reduction of the anterior 

 and posterior laterals with regard to number and strength ; 

 on the other hand, the cardinals have strongly developed 

 with regard to their thickness, though there is unquestion- 

 ably a tendency towards the reduction of number by resorption 

 of the posterior cardinals of higher order. 

 " II. The second group comprises the genus Mactra. In this genus 

 just the reverse takes place as in the first group ; the anterior 

 and posterior laterals are strongly developed with regard to 

 numljer and strength ; on the other hand, the cardinals are 

 almost rudimentary, while their number is greatly reduced. 

 "III. The third group is represented by the genus Meiocardia ; in 

 this genus laterals and cardinals are neither reduced in 

 number, nor has the strength of one been increased at the 

 expense of the other ; the peculiar feature is that originally 

 separate teeth, originating from different primary lamelke, 

 have become amalgamated and form composite teeth, which 

 hardly allow their primary elements to be traced. An 

 originally complex hinge has therefore become simplified not 

 by disappearance of some of its elements, but by amalgama- 

 tion of some of them, a feature which has not been noticed 

 in either of the two preceding groups." 

 Au Appendix (pp. 44-57) on the Variability of Pelecypod Shells 

 suggests a useful " Index " of numerical value, with the figure or 

 formula L/H (length and height placed over the average measure- 

 ment, thus — 1^/3145) ; and its application, especially with the 

 graphical method of diagram, in which, the numerical values being 

 grouped in a horizontal line, their heights can indicate the curve of 

 variability in a given species (see plate v.). 



PROCEEDmGS OF LEARNED SOCIETIES. 



GEOLOGICAL SOCIETY. 



March 12th, 1902.— Sir Archibald Geikie, D.C.L., LL.D., F.R.S., 

 Vice-President, in the Chair. 



The following communication was read : — 



' On Proterozoic Gasteropoda which have been referred to 

 Mui'chisonia and Pleurotomaria, with Descriptions of New Subgenera 

 and Species.' By Miss Jane Donald. 



Jilany of the Palaeozoic shells referred to Murchisonia do not 

 agree with the type, and there are at least two separate groups 

 distinguished by the outer lip. The typical group has a slit, the 



