Manchester Memoirs^ Vol. Iv. (1911), No. 8. 15 



The typical Unionid teeth consist of much elongated 

 posterior laterals and anteriorly placed antero-laterals, 

 either without any buttress or with only a weak one. 



The extreme length of the posterior laterals is pro- 

 bably connected with their greater post-umbonal develop- 

 ment owing to their habitat in a region of continuous 

 current. The change from typical Trigonid to typical 

 Unionid teeth can be bridged over by American forms : — 



(i) U. ellipsis has sub-equal ridged teeth with the 

 anterior strongly buttressed. 



(2) Tetf-aplodon ainbiguus ( = Cast alia [Lam])[Pernam- 



buco] has a somewhat elongated posterior lateral 

 and an anterior buttress. 



(3) Quadrula trigona {=U. trigonus [Lea]) has typical 



Unionid teeth, but with a strong buttress. 



(4) Pleiirobema bigbyensis {= U. bigbyeiisis [Lea]) has 



teeth as in Quadrula, but with a much reduced 

 buttress. 



2. The original description of Trigonias was based on 

 their form, with two triangular areas, the smaller, the "area" 

 being posterior. Modern adult British Unionidae show 

 no such distinction on their valves. It is seen, however, 

 in Q. lachrymosa, and in the S. E. African forms. It 

 is distinguishable on the umbones of the British and 

 American forms. The ornament in the two groups is, I 

 believe, similar, but this point requires further working 

 out. 



The great differences in structure, viz. : — 



'(i) the entire absence of a lunule in Trigonias ; 



(2) the presence of an ant. buttress in „ 



(3) the absence of an accessory pedal scar in „ 

 can be shown to be due to progressive variation, and can 

 be bridged over by intermediate forms. 



