THE SHELLS OF THE GULF OF CALIFORNIA. 



20! 



of the Pacific coast of America, representing there the very distinct* 

 C. fornicata of the Atlantic. Two extreme forms were first described 

 by Broderip, from Mr. Cuming's collection: the one, G. squama, thin, 

 flat, and smooth; the other, C. lessonii, solid, often arched, and cov- 

 ered with concentric laminae. These sometimes appear at regular 

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

 shell. It appears, however, that C. 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 creature advances forward, turning 

 up the previous portion, the form Lessonii is produced: in general 

 very roughly and irregularly, which is the C. 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 types, 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 C. 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 condition, intermediate between the 

 extremes first described, is the C. nivea of G. B. Adams. As it is 

 the normal state, the usual rules of priority have been set aside, and 

 C. nivea taken for the name of the species, leaving squama and Les- 

 sonii for the principal varieties. The White Slipper is known under 

 all forms (when in good condition) by its shaggy, light-green skin, 

 and by the very peculiar character of the nuclear whirls. These are 

 remarkably small, though the shell is large, standing out from the 

 surface, of a reddish tinge, and crowded with regular transverse ribs. 

 The characters have 

 been observed in 

 specimens of all the 

 forms, although the 

 influences which pro- 

 duce Lessonii, draw- 

 ing the shell away 

 from the vertex, gen- 

 erally lead to its ab- 

 rasion. Sometimes 

 the White Slipper 

 goes to live, when 

 young, in the empty 

 burrow of a boring 

 mussel. In these 

 cases, as soon as it 



has grown to the Ciepidula nivea, jun.— Outside, a, nuclear spiral portion, ribbed; h, », 

 • nv r •■ _ ^ margin of deck, seen through the transparent shell. 



c It does not follow, because certain aberrant forms from different localities resemble 

 each other, that the species arej therefore identical, if the normal state and general habit 



