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(as it generally is in elongated specimens), it can hardly be dis- 

 tinguished from similar specimens of the White Slipper. But 

 as soon as it has reached the bottom of the pipe, where the 

 dead bivalve (generally a Petricola, a creature with rather short 

 siphons), still remains undecomposed, it suddenly encounters an 

 unexpected obstacle. It wedges itself under this (to it) mighty 

 globe, and turns its delicate mantle, exuding the shelly skin up 

 the sides of the cavity, but in vain. There is nothing for it but 

 to retrace its steps, and back out. As it does so, every new 

 portion formed under the arched bivalve repeats the previous 

 concave impression, and the grooved slipper is the result. The 

 sharp instrument of the explanation of one author, and the 

 " perforation " of the other, is nothing but the little rounded 

 u clam " tightly wedged at the bottom of its burrow ; and the 

 same slipper-limpet freely developed under unconstrained influ- 

 ences, is probably the C. navicelloides of Nuttall, to ascertain 

 the characters of which we are still in want of perfect specimens. 

 " To return to the White Slipper on the back of our Thorn 

 Oyster. Among the young shells which appear to the naked 

 eye to be the young C. nivea, were some which under the micro- 

 scope displayed a much larger but smooth and imbedded nuclear 

 portion. On comparing these with similarly situated specimens 

 from the west coast of Africa and from other places, I found 

 them exactly identical. They probably belong to the C. ungui- 

 formis of Lamarck. Now, it so happens that Prof. C. B. Adams, 

 who in general described every shell of Atlantic types as a new 

 species, if found on the Pacific coast, in this one instance felt 

 constrained to adopt the I^amarckian name for the unguiform 

 Slippers of Panama. It is not certain that in this one instance 

 he was correct. Some of the specimens he distributed under 

 the name are undoubtedly compressed and inverted forms of his 

 own C. nivea ; for every species may take the form of ungui- 

 formis when grown inside of a dead spiral shell, especially with 

 a dead hermit crab pressing against it. But there seems suffi- 

 cient evidence to believe that while each coast has its special 

 species of slipper-limpets, each one of which assumes protean 

 changes, there is in this one species which has been scattered, it 

 may be in dead shells, and on ballast, round the world, and to 

 be distinguished from all neighboring species by the peculiar 



