DALL : MOLLUSCA AND BRACHIOPODA. 231 



CavoUna teJemus A. Adams, Ann. Mag. N. Hist., 1859, 3d ser., 3, p. 44. 

 Cavolinia tridentata Fischer, Man. de Concliyl., 1883, p. 434 ; Pelseneer, Chall. Rep. 

 rter., 1887, p. 83. 



Station 3407, U. S. S. "Albatross," in 885 fathoms, near the Galapagos Islands. 



This species was obtained by the " Albatross " in 1887 at several stations in tlie 

 South Atlantic off the coast of Soutli America and is widely distributed in the 

 open sea of both the Atlantic and Pacific oceans. A long list of stations is given 

 by Boas, and Arthur Adams reports it common in the Atlantic and Indian oceans. 



The tendency of the early authors who had small collections of pteropods was 

 to name each distinguishable form, and doubtless far too many names were pro- 

 posed. The reaction from this has swung the pendulum in the other direction 

 perhaps too far, and it is probable that a more thorough knowledge of the living 

 animals would sliow the truth to lie between these extremes. The uniformity of 

 the living animals taken from a single swarm, and the wide differences of form, 

 color, and proportion which appear in the sketches made from life, can hardly all 

 be attributed to errors of the draugiitsman, thougli the latter must also be allowed 

 for. It is the writer's opinion that future investigation will show that there are 

 some species of Cavolina, at least, which have very similar tests, but differ in form 

 and color of tlie flippers and other parts of the body. 



Tlie fall illustration of the stages of growth in the various thecosomatous ptero- 

 pods is yet a desideratum. 



The unequal contraction of muscular and cellular tissue in alcohol, whicli is 

 exceptionally great among the pteropods, prevents the student of alcoholic mate- 

 rial from gaining any just idea of the proportions of the living animal, while its 

 rich violet or crimson coloration is almost wlioUy lost. 



Dr. J. I. Peck discussed the pteropods of the " Albatross " voyage around the 

 Horn, but the very few collected, and the small number of stations, render gen- 

 eralizations on liis material of very little weight. 



The rapid dissolution of the fragile pteropod shells under tlie influence of the 

 carbon dioxide contained in sea water at considerable depths is absolutely cer- 

 tain, and unless the dead shells were almost immediately recovered by the dredge, 

 no trace would remain of them. It is probable that a very few months are all 

 that is needed to completely dissolve the shell of a Cavolina or CIcodora. In 

 regions where there is a considerable quantity of organic lime-material on the 

 bottom, as off St. Augustine, Florida, in 400 fathoms, the small pteropod shells 

 are well preserved, but in the deeper and more argillaceous areas they are found 

 with extreme rarity, though abundant on the surface. Hence little can be safely 

 predicated from the abscMice of pteropod shells on a given bottom, and it is abso- 

 lutely unsafe to base generalizations of distribution upon negative evidence of this 

 kind. 



The season of the year and the state of the great oceanic currents has much to do 

 with the abundance and even the presence of pteropods. In the Pacific the writer 

 has made some twenty-six voyages between California and the .\lcutian Islands, 

 on nearly all of which a tow net was kept going. Consequently a tolerably full 



