ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 61 



Deep-sea MoUusca.* — In Lis report on the Bracliiopoda and Pele- 

 cypoda collected in the Gulf of Mexico and in the Caribbean Sea, Mr. W. H. 

 Dall has an interesting introduction on the deep-sea forms. Large shells 

 appear to be rare at great depths, and when found are very fragile ; the 

 tissues are loose and gelatinous, owing probably to the necessity of afford- 

 ing that thorough permeation of the tissues which is necessary to equalize 

 pressure. Almost all the shells are extremely thin and light. The colours 

 are faint or delicate, the iridescence often peculiarly brilliant. Many 

 abyssal shells have a delicate and sometimes profuse sculpture. 



Mr. Dall points out that the word " deep " has had many significations ; 

 the abyssal area is that in which the temperature of the bottom is known 

 to be quite uniformly cold, where the food cannot vary much in quality 

 or quantity, and where the distribution of life is comparatively sparse and 

 uniform ; he applies the term of archibenthal area to the continental region 

 of Prof. A. Agassiz ; it is that which lies between the littoral and the 

 abyssal. It is in this that the chief treasures of the dredger are to be 

 found ; the littoral and archibenthal faunse are often entirely or almost 

 entirely dissimilar, but in the far north or in the tropics the species may be 

 found in shallow water of the appropriate temperatures. 



The bottom of the ocean is generally composed of fine impalpable mud, 

 and there are no stones or rugose inorganic objects for sedentary molluscs 

 to perch on ; the tubes of hydroids and annelids or the long spines of sea- 

 urchins may afford the necessary points d'appui. 



The author urges that naturalists do not seem to have realized that 

 natural selection " may act, in certain cases, as successfully by confining 

 the inflexibility of a particular stock, as it does in others by seizing the 

 favourable variations of the vast majority of living beings, which vary 

 indefinitely in all directions." It is probable that the few molluscs which 

 have been recognized as having a world-wide distribution owe their uni- 

 formity to some such cause as this. The abyssal molluscs are nearly all 

 flesh-eaters, and as they get their food from the constant gentle rain of dead 

 or dying animals, they are not compelled to prey much upon one another. 



As it seems to be a general law of animal structures that the greater 

 the number of similar parts in any member of an organic individual, or of 

 similar members, the greater the tendency to vary, first in the minor features 

 of these parts as compared with each other, and secondly in the number of 

 similar parts in any individual as compared with the average number 

 characteristic of the species ; and as in the deep sea the factors which affect 

 the tendencies to vary — absence of light, of enemies that can see, of violent 

 motion — are almost eliminated, " the logical result is that we may expect 

 in the deep sea a very wide range of variation in form and sculpture within 

 the specific limits of the ' flexible ' species, and an almost complete uni- 

 formity over very wide areas of the forms which we may consider as 

 inflexible species." And this, in Mr. Ball's judgment, is what is actually 

 found. 



Of the groups of MoUusca described in this report there are 227 

 species or varieties, 81 of which are new ; there are 12 new subgenera or 

 sections. 



* Bull. Mus. Comp. Zool. Cambridge, xii. (1886) pp. 171-318 (9 pis.). 



