10 THREE CRUISES OF THE " BLAKE." 



The bathymetrical range of the mollusks is also connected 

 with a wide geographical extension. According to Mr. Ball, if 

 we consider the species dredged from the Atlantic Ocean north 

 of a line drawn from Hatteras to Madeira, by all expeditions up 

 to 1883, at greater depths than one thousand fathoms, we find 

 that more than forty-two per cent live in some locality in less 

 than one hundred fathoms. 



These species of mollusks have apparently taken advantage 

 of the uniform conditions of existence in deep water, and have 

 extended their range far from their original littoral abode. 

 There is a tolerable number of species, evidently unchanged, 

 which occur all the way from a few fathoms, on the Florida 

 coast, to two thousand fathoms in the adjacent deeps. A better 

 knowledge of the littoral fauna of the tropics would undoubt- 

 edly increase this percentage. We also notice that the per- 

 centage of the genera or families peculiar to the continental and 

 abyssal regions is small. 



The sea-urchins and starfishes have their fullest develop- 

 ment in the continental zone, and there we find already many of 

 the genera and families which have given so characteristic an 

 aspect to the fauna of deep waters. Beyond that region live 

 the eminently deep-sea types of the Pourtalesia3 and Ananchy- 

 tidaB, associated with a few starfishes and the strange order of 

 holothurians, the Elasipoda. The ophiurans appear, of all the 

 echinoderms, to flourish best in the deepest waters from which 

 members of the class have as yet been dredged. The bathy- 

 metrical range of many of the sea-urchins and ophiurans is very 

 great, and extremes of depth extending to two thousand fath- 

 oms or more are not uncommon. 



The stalked crinoids, as has been shown by Carpenter, are not 

 strictly abyssal types ; on the contrary, seventy-five per cent of 

 them have been brought up from depths of less than five hun- 

 dred fathoms, somewhat deeper than the limit of the conti- 

 nental zone. As stated by Carpenter, out of the thirty-two 

 recent species of stalked crinoids, nine species may be called 

 littoral, living as they do at depths of less than one hundred 

 fathoms. 



Comatulse were dredged at fifty-seven out of the two hundred 



