Sj)on(/e4i from t/ie Atlantic Ocean. 475 



but often representing casts, witli their peculiar markings, of 

 the clianibers of (rfofn'tjerina and otlier minute Foraminifcra. 

 If a little of this sand be carefully washed, dried, and placed 

 under the microscope, it will be easily seen that they have 

 all the same origin ; for, beginning of a yellowish colour, 

 passing into brown, and finally l)lack, thoy may respectively 

 be observed within the chambers of (ilohujcrina, half in 

 and half out, as they ap[)roach that state in which, being 

 altogether without even a fragment of the white calcareous 

 test, and in the form of casts, they either retain this recog- 

 nizable form or lose it altogether and become more or less 

 angular. 



EXPL.\NATIOX OF TILE PLATES. 



Plate XIL 



Fiij. 1. Pebble on which there is a Terebratule and six kinds of sponges. 

 a rt, pebble ; h, Terebratule, — the Terebratule bearing- c, Ajjli/siita 

 ncfvu!*, d, Sj)(iu(/ia ojficinalis, e, Dysidia f)-<it/i/is ; the pebble 

 bearing:—///, fronds of two specimens of rhukvUia vnitiluhrum, 

 Bk. ; y (j, >Sp()tit/ia Lbvenii, Boo. ; h, Microciona Imigispiculum. 

 All natural size. 



Fitj. '2. Aphjsiiia va>vus, grown over a branch of coral, natural size. 

 a, fragment, magnified, to show : — i, dermal incrustation covered 

 with jiore-depressions or puncta ; and c, basal end of filameuta 

 expanded into layer of attachment. 



Fiy. 3. Dictijucijliudrus abt/ssorioii, natural size, a, portion of branch, 

 magnified, to show hirsute character; b, small acuate, spined ; 

 c, anchorate ; d, tricurvate (bow) : b, c, d on scale of l-24th to 

 l-1800th of an inch, e, end of tricurvate, more mafrnified, to 

 show that it is spined ; f, anchorate, more magnified. For 

 skeleton-spicules see PI. XV. fig. 25, a, b. 



Fiy. 4. Ilymeruphia venniciduta, Bk., var. erecta, natural size, rt, fixed 

 end of large skeleton-spicule ; and b, tortuous subskeleton-spi- 

 cules with which it is surrounded : scale l-24th to l-lf>00th 

 inch, c, II. vermiculatd, Jik., covering a small pebble : natural 

 size. For skeleton- and subskeleton-spicules, see PI. XV. 

 fig. 20, a, b. 



Fiy. o. Didyncylindrus viryxdtostis, Bk., bearing a young Tethya craniuin 

 and a ^ ariety of Cirautia ciliota. a, Tethya ; b, Oraidiu ciliida ; 

 c, small acuate spined spicule of Z>. virytdfo.vt.'i; d, act-rate sub- 

 skelfton-spiculc, smooth : .scale of c, d, l-24th to 1 -1800th inch. 



Fiy. C. Orantid ciliufa, Flem., vai". spinispiculmti, C, on Didyucylindrus 

 virytiltusu.t, natural size. 



Fiy. 7. The same, barbed spicule among the acerates towards the base : 

 (I, tixed end ; b, free or barbed end. Scale l-24th to 1-OOOOth 

 iiich. 



Fig. 8. Grantia cHiata, Firm., variety (from Budleigh-Salterton, south 

 coa.st of Devonshire). Two barbed sjiicules with inflated extre- 

 mities, respectively ; one spined like the anclioring-spicule of 

 Fiijdcctella aaprryillum. 8cale l-12th to l-(>000th inch. 



Fig. 9. Cornidinii textile, natural size. «, textile sheath ; b, tibruus struc- 



32* 



