^^] INSTRUCTIONS FOE COLLECTING MOLLUSKS DALL. 



.; MARHsTE SPECIES. 



''k.s, everyone is aware, the sea is the most prolific region for molhis- 



xsaQ life, far exceeding in the number of its species the land and fresh- 



w.^ter regions combined. A similar disproportion exists between the 



pective numbers of families and genera. The earliest known mol- 



ks were coeval with the earliest fossiliferous rocks and were marine 



?ms. Air-breathers are not known to have existed before the Car- 



niferous period, but when the much more ancient Cambrian forms^ 



•3 living the molluscan type was already old and exhibited develop- 



;it in several of its principal lines. There can be no doubt that the 



i, has continuously existed since the earliest development of life on 



globe, and most naturalists believe that in it the first organic life 



k rise. Marine mollusks, regarded as a whole, have therefore formed 



ontinuous series, and in the depths of the sea are to be sought those 



= ecesses where change of conditions from age to age has remained at 



■'^ minimum. There linger forms which are of incalculable antiquity, 



ne of which differ little, regarded as generic types, from some of those 



Jch existed in Paleozoic time; while representatives of genera devel- 



ed in Oenozoic time are numerous. 



REGIONAL DISTRIBUTION OF SPECIES. 



The existing sea, with reference to its molluscan population, is divis- 

 ■ Ijle into two sorts of regions. One set is most easily defined as that 

 of areas differing by differences of latitude, the other by differences in 

 depth of water. These differences in either case are not absolute 

 • ut relative, depending on temperature and food supply, but, in a geu- 



al way, we are not inaccurate when we speak of Arctic, Boreal, Tem- 

 urate, Subtropical, and Tropical mollusk-faunas, or those of the shores, 

 he shallows, and the abysses. 



The waters immediately adjacent to the shores were long ago divided 

 iy Forbes and other pioneers in marine exploration into zones or areas 

 according to the conditions characterizing them; as, for instance, the 

 ' aminarian zone or region of brown kelp, the Coralline zone or region 

 .>f stony algse, etc. But for general purposes and to contrast the areas 

 jf the whole sea, one with another according to their chief characteris- 

 ■ics, we may divide the entire sea bottom into three regions. 



The first is that to which light can penetrate and therefore where 

 marine vegetation can exist. This is the Litoral region, and in a gen- 

 eral way, modified by especial conditions at particular places, it may be 

 regarded as extending from the actual shore out to the limit of 100 

 fathoms. Beyond this it is practically certain that no light reaches 

 the bottom of the sea and no sea weeds grow. Outside of this the bor- 

 ders of the continents slope gradually to the bottom of the ocean, which 

 is found usually at a depth of about 2,500 fathoms. 



On the upper parts of these continental slopes the conditions are often 

 very favorable for marine life. Currents of comparatively warm water, 



