'm*"'] PROCEEDINGS OF THE NATIONAL MUSEUM. 229 



not been developed also iu shallow-water forms and for similar reasons. 

 The distinction which I would point out is that in litoral species, as a 

 rule, these devices are subsidiary to the much simpler course of strength- 

 ening the shell by adding to its thickness. In the abyssal forms, for 

 reasons already explained, this mode is not practicable and consequently 

 we have the one without the other. .The operculum is generally horny 

 in abyssal mollusks, frequently disproportionately small, compared with 

 that of congeneric litoral species, and iu a remarkably large number of 

 cases is absent altogether. 



As might be expected of descendants with modification, the resem- 

 blance is greater between the larval shells of ben thai species and those 

 of their shallow-water relatives than between the parts of the shell of 

 later growth. There is one notable difference, however. In the deep- 

 water forms the nucleus is frequently larger thau in their litoral ana- 

 logues. It would seem as if the condition of the depths were such that 

 of a small number of large larvae more are more likely to survive 

 than of a large number of small ones ; or at least that this form of repro- 

 duction is more useful to the species. These details will serve to show 

 the multiplicity of facts to be accounted for and the opportunity for 

 advancing science by a study of abyssal conditions and their effects 

 upon the animals subjected to them. Without claiming any unique 

 importance for the theories advanced in the foregoing remarks it 

 may still be said that the subject is one of the very greatest interest. 

 Perhaps experiments upon shallow-water forms, artificially subjected 

 to pressure, may at some future time enable us to penetrate more 

 deeply into the mysteries of life in the abysses. 



It now remains to take up the collections made by the Albatross party 

 on their voyage. 



Beginning the enumeration at Santa Lucia iu the West Indies, and 

 terminating it at San Francisco, California, it appears that the register 

 of operations includes one hundred and forty dredging stations and 

 forty anchorages, besides sundry surface collections. From the inspec- 

 tion of the collection of mollusks, which is almost entirely preserved 

 in alcohol, it seems that mollusks were collected in eighty of the casts 

 of the dredge or trawl, and at twenty-seven of the anchorages, distrib- 

 uted as follows: 



On the Atlantic coast of America and in the Straits of Magellan, six- 

 teen stations and eleven anchorages are represented, of which eight 

 casts were in water over 100 fathoms deep. 



On the Pacific coast of South America from the Straits of Magellan to 

 Panama and to the Galapagos Islands mollusks were collected at 

 thirty-nine stations and twelve anchorages. At eleven American sta- 

 tions and three near the Galapagos Islands the depth was over 100 

 fathoms. 



On the Mexican and Central American coast north of Panama mol- 



