110 CALYPTPLEID^. 



cific variation. These extreme forms are very naturally described 

 as distinct species, the intermediate connecting links not passing 

 before the view of the naturalist. On showing to a distinguished 

 author a carefully eliminated suite of Mazatlan specimens con- 

 necting the smooth, thin, flat Grepidula squama, Brod., with 

 the coarse, arched, laminated C. Lessonii, passing through the 

 forms C. nivea, C. B. Ad., and C. striolata, Mke., he complained 

 that I had ' kept all the puzzling shells.' In the very useful work 

 of Messrs. H. and A. Adams, on the ' Genera of Recent Mol- 

 lusca,' these forms appear under different subgenera. It is not 

 fair to blame authors for these mistakes, which naturally result 

 from the imperfection of the material on which they work. 

 But the prevalence of such errors should lead us to embrace 

 every opportunity of studying large numbers of specimens, both 

 from the same and different localities. Patience, accuracy, and 

 honesty may thus render as valuable service to science as bril- 

 liant genius, and may supply the materials from which some 

 master-mind may hereafter develop the most important generali- 

 zations. 



" Those who describe species from minute differences founded 

 on individual specimens, might do well to study the plates ap- 

 pended to the 'B. A. Report on the West Coast Mollusca.' 

 Take e. g. the Crucibulum spinosum, pi. 9. The shell is at first 

 spiral, like a snail. It then surrounds its entire margin with a 

 rim, which is the first beginning of what in the adult becomes 

 the ' saucer ' or outside shell ; that is the hardened skins of the 

 animal's body (for the shells are not to be regarded as a house, 

 constructed for the animal to live in, but as an integral part of 

 the animal itself, like the feathers of birds or our own nails and 

 hair). At the same time it raises a slight lamina from the labi- 

 um or ' pillar-lip ' which ultimately becomes the ' cup/ At first, 

 however, it is like the ' deck ' in the Slipper limpets, from some 

 species of which it can scarcely be then distinguished. The 

 Crepidulae, however, continue their deck in a horizontal direc- 

 tion, while the Crucibulum turns the edges upwards at a more 

 or less obtuse angle. Gradually during the progress of adoles- 

 cence, this angle becomes right and then acute, the outer shell 

 meanwhile taking various forms, round, oblong, or irregular, 

 according to the nature of the surface to which it has chosen to 



