Sponges from the Atlantic Ocean. 475 



but often representing casts, with their peculiar markings, of 

 the chambers of Glohigerina and other minute Foraminifera. 

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

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

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

 passing into brown, and finally black, they may respectively 

 be observed within the chambers of Glohigerina^ half in 

 and half out, as they approach 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. 



EXPLANATION OF THE PLATES. 



Plate XII. 



Fig. I. Pebble on which there is a Terebratule and six kinds of sponges. 

 a a, pebble ; h, Terebratule, — the Terebratule bearing c, Aplysina 

 navus, d, Sjjunr/ia officinalis, e, Dysidia frcu/ilis ; the pebble 

 bearing:—///, fronds of two specimens of i%f/^eZ/ia ventilahrum, 

 Bk. ; g g, Spongia Lbvenii, Boc. ; h, Micruciona longispiculum. 

 All natural size. 



JFig. 2. Aplysina nccvits, grown over a branch of coral, natural size. 

 a, fragment, niagnitied, to show : — h, dermal incrustation covered 

 with pore-depressions or puucta ; and c, basal end of filaments 

 expanded into layer of attachment. 



Fig. 3. Didyocyliiidnis abyssurum, natural size, a, portion of branch, 

 niaguitied, 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 magnified, to 

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

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



Fig. 4. Hymeraphia vermicidata, Bk., var. erecta, natural size. «, fixed 

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

 cules with which it is surrounded : scale l-24th to l-1800th 

 inch, c, H. vermicidata, Bk., covering a small pebble : natural 

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

 fig. 2G, a, b. 



Fig. 5. Dictyocylindrus virgultosus, Bk., bearing a. young Tethya cranium 

 and a variety of Grantia ciliata. a, Tethya ; b, Grantia ciliata ; 

 c, small acuate spined spicule of D. virgtdtosus ; d, acerate sub- 

 skeleton-spicule, smooth : scale of c, d, l-24th to l-1800th inch. 



Fig. 6. Grantia ciliata, Flem., var. spinispicidwn, C, on Dictyocylindrus 

 virgidtosus, natural size. 



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

 a, fixed end ; b, free or barbed end. Scale l-24th to 1-GOOOth 

 inch. 



Fig. 8. Grantia ciliata, Flem., variety (from Bndleigh-Salterton, south 

 coast of Devonshire). Two barbed spicules with inflated extre- 

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

 Eiqdectella asj)ergilhim. Scale l-12th to l-6000th inch. 



Fig. 0. Cormdum textile, natural size, a, textile sheath ; b, fibrous struc- 



32* 



