114 C ALTPTR^ID^. 



intermediate forms of variation are found in the same locality, 

 the changes depending on the accidents of individuals, it is not 

 yet proved that the idiosyncrasies are transmitted. In fact, the 

 frequent instances in which the individual itself changes its 

 form and sculpture at different periods of its life, is against 

 such a hypothesis, 



" It is a fact worth noticing that while some species of shells 

 are extremely variable, others, inhabiting the same localities, are 

 very constant in their characters. These are seldom widely 

 diffused, and are often rare in individuals. A few young speci- 

 mens of such species were found among the slipper-limpets on 

 the spondyli ; but the bulk of the specimens belonged either to 

 C. aculeata, which, as we have seen, is a somewhat ubiquitous 

 species, or to G. nivea, which, under many shapes and many 

 names, spreads over the principal parts of the Pacific coast of 

 America, representing there the very distinct G.fornicata of the 

 Atlantic. Two extreme forms were described by Broderip, from 

 Mr. Cuming's collection ; the one, C. squama, thin, flat, and 

 smooth; the other, G. Lessonii, solid, often arched, and covered 

 with concentric laminae. These sometimes appear at regular 

 intervals, and seem to be the normal and unique sculpture of the 

 shell. It appears, however, that G. squama (which is the calm- 

 water form), if exposed to rougher influences, arches its back, 

 adds layer after layer of porcellanous matter, hiding the color 

 rays, and leaving the margin like the edge of a quire of paper. 

 Now, if co-ordinate with this laying on of extra coats, the crea- 

 ture advances forward, turning up the previous portion, the form 

 Lessonii is produced ; in general very roughly and irregularly, 

 which is the G. striolata of Menke, but sometimes very delicately, 

 with fine sculpture between the laminae as described by Brod. It 

 is common to find shells living for some time as squama, and 

 suddenly plunging into the Lessonii tj^pes, with one or two 

 strong laminae. Every stage of intermediate form was found 

 among the Mazatlan shells. The degraded specimens of the 

 Chilian seas form a part of the G. protea of d'Orbigny^a 

 convenient receptacle, as the type specimens in the British 

 Museum show, for the dead and puzzling shells which the 

 author did not know where else to place. The ordinary condi- 

 tion, intermediate between the extremes first described, is G. 



