SUBALPINE MOLLUSCA. 79 



To show Still further that Prime's types are not the 

 usual forms, but extreme varieties, it is noticeable that 

 other authors give less different measurements, as follows : 



Length. 

 S. partumeitxra Saj/ (type) .... 0.55 



S. partumeium Gould 0.45 



S. securis Gould 0.33 



S. truncatum Gould (type) .... 0.33 

 S. lenticula Gould (type) 0.40 



By drawing diagrams from these figures, the outlines of 

 the four forms are found much more alike than those of 

 Prime, the diameters also being closely like those in our 

 plate. 



To show that young shells of the rhomboid group do 

 not much resemble that of S . rayiuondi, figs, 5 and 24 

 are comparable. The variations of the latter species 

 from age, etc., show how much should be allowed for 

 variation in other species. 



In comparing our figures with those given of the same 

 species in Prime's Monograph of CorbiculadcE, it will be 

 noticed that the latter give the impression that the shells 

 are much more angled in outline and in the umbonal ex- 

 pansion than the photograph shows them to be. While 

 there can be no doubt of the accuracy of photographs, it 

 may be said that ours do not represent fully adult shells. 

 While admitting this as to some Eastern species, we show 

 that the Western species {S . lenticula, fig. 11) is larger 

 than Prime's figured type, and yet it does not exhibit any 

 better the angulation and truncation given in his figure 

 more strongly than in any of the allied forms. His artist 

 or engraver, making small wood cuts, apparently cut the 

 distinctive features rather more strongly than they exist 

 in nature. 



In many cases the figures and descriptions do not agree 

 even as well as would be explained by allowing variations 



