Manchester Memoirs, Vol. Iv. (191 1), No. 8. 



VIII. Studies in the Morphogenesis of certain Pelecy- 

 poda. (i) A Preliminary Note on Variation in 



U/iio pictorinn, Uiiio tiimidiis and Aiiodonta cygnea. 



By Margaret C. March, B.Sc 



(Comimcnicated by Dr. G. Hickling.) 

 Read December i^tli, igio. Received for publication January i-jih, igii. 



In studying the British fresh water Unionidae, 

 Anodonta cygnea, Llnio pictorum and Unio tmiiidus, with a 

 view to ascertaining the amount of variation in form in a 

 well defined modern species, several facts of general 

 application became clear. 



Relation bctzvccn variation in form and environment. 



I. From the study of the form of U. pictorum and U. 

 tnmidns from the collections of Mr. Standen, the late 

 Mr. R. D. Darbishire, Mr. J. W. Jackson, and the Concho- 

 logical Society it became evident that there were two 

 main types of shell, the first stout and heavy with relatively 

 long dorso-ventral and lateral axes and short antero- 

 posterior axis. {Fig- A of Plate.) The other form was 

 the antithesis of this. 



A further indication of the existence of such forms 

 was obtained by variation curves, taking antero-posterior 

 or lateral axis in relation to the dorso-ventral, Although 

 the numbers were too few to give a definite proof (132 

 U. pictorum, 7S U. tumidics, 114 A. cygnea) yet they gave 

 an indication of the presence of two such forms. 



March 14th, igii. 



