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LECTURES ON 



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

 nivea, C.B. Ad. and C. striolata, like., 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 Mollusca, these forms appear under 

 different subgenera.* It is not fair to blame authors for these mis- 

 takes, 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 from different localities. Patience, accuracy, 

 and honesty may thus render as valuable service to science as brilliant 

 genius, and may supply the materials from which some master-mind 

 may hereafter develop the most important generalizations. 



Those who describe species from minute differences founded on in- 

 dividual specimens, might do well to study the plates appended to 

 the "B. A. report on the West Coast Mollusca"" before quoted. 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 skin of the animal's body; (for 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 

 labium, ov "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 Crepidulas, 

 however, continue their deck in a horizontal direction, while the Cru- 

 cibulum turns the edges upwards at a more 

 or less obtuse angle. Gradually, during the 

 progress of adolescence, this angle becomes 

 right and then acute ; the outer shell mean- 

 while taking various forms, round, oblong, or 

 irregular, according to the nature of the sur- 

 face to which it has chosen to adhere. Often 

 this immature state is continued to a late 

 period; if permanent, it would belong to the 

 subgenus Dispoteea (Say) of Messrs. Adams. 

 But, normally, the sides of the cup close in, 

 while its body becomes greatly swollen in 

 front. This cup now assumes the form which 

 is always characteristic of the species under every modification of ex- 

 ternal growth ;' being well rounded in C. imbricatum, angular at the 

 side in C. spinosum, and with the sides flattened against each other 

 in C. radiatum. In C. rude, the adolescent stage is very soon com- 

 pleted, and the cup is permanently detached from the side of the 

 shell, forming a veritable "cup and saucer," one, too, after the fashion 

 so prevalent in America, where the cup-handle has never been formed. 



*The plan adopted by D'Orbigny in his classification of Foraminifera, was to pick out 

 from a large mass of material the leading forms ; which he grouped into genera, families, 

 and orders. In my brother's papers on Orbitolites, &c , in the transactions of the Royal 

 Society, it is shown that individuals belonging, according to D'Orbigny, to different orders 

 are really aberrant forms of the same secies. 



Crucibulum imbricatum, jun. 

 Interior view, showing the cup lamina 

 beginning to double in. 



