BULLETIN 50, ITNITED STATES NATIONAL MUSEUM. 



Table .thon'irKj the numerinil distrihid'toa nf formn aceordingto depth. 



37 



l>e|itli in fathimis 



Number of forms 

 found at that 

 ilt'[)tli 



Number of forms 

 confined to that 

 depth 



Xuinlipr of forma 

 rantiinfj into the 

 next deeper 



Number of forms 

 occurring in next 

 shallower 



0-25 



Number of forms 

 occurring in both 

 shallower and 

 deeper water 



25-40 



14 



/ 40- 100- 

 \ 100 I 200 



200- 

 800 



21 



14 



300- 

 400 



400- 

 500 



500- 

 600 



600- 

 700 



1" 



700- 

 800 



800- 

 000 



900- 

 1,150 



a This species was alsa collected between 800 and 900 fathoms. 



The preceding table is not absolutely correct, because of the insufficiency of the 

 data regarding the depth at which some of the forms were collected. As some of 

 the species are divided into "'varieties" or "'forina''" the problem of comparison is 

 additionally complicated. The following general conclusions are plearly indicated: 



The greatest numlier of forms are developed in shallow water, between and 2.5 

 fathoms. There are over five times as many forms between and 2.5 fathoms as 

 between '25 and 40. Forty fathoms represent the maximum depth to whicii any 

 reef species extends. There is rapid further decrease between 40 and lOO fathoms, 

 the ratio to the number occurring between and 25 fathoms being 1:11; between 

 25 and 40 fathoms, 1:2. Only 2 forms are peculiar to this depth, the others ranging 

 into shallower or deeper water, while one ranges into both shallower and deeper. 



The number of forms increases between 100 and 200 fathoms; the ratio to the 

 number between 40 and 100 being 3:1. Between 200 and 300 there is a marked 

 decrease; between 300 and 400 a still further decrease. P'rom 100 to 400 fathoms, 

 iiowever, seems to be a second faunal zone. 



As this is considered a very important zone, the fornis occurring in it are relisted. 



FORMS OCCURRING BETWEEN loo AND 41)0 FATHOMS. 



Flahelluiii jxionniniiin Lesson, typical. 



JF. pavoninum var. dintinctiuii Milne Kdwards and llaimc. 



F. pavonlnuin var. pari]>aroninuin Ak^ock. 



F. j)avu)i.intun. var. lafimi. Stiider. 



Placotrochus fmcus Vaughan. 



Gardincria hawaiienxlx \'aughan. 



DesmophyUuia crlstagaUi Milne Edwards and Ilaime. 



