PLATE XXIII. 



FIQ. 



1. — A Stephanastruni, with the four ceutral arms similar to those of 

 Stephanastriim rhombus of Ehrenberg, enveloped in a circular 

 spongiose web, beyond which extends an irregularly-spiuous 

 transparent siliceous border, which also extends round the ends 

 of the arms. I think this round web is the normal form of the 

 Stephanastrums and Khopalastrums, and the key to all the 

 curious 3, 4, 5, and 6-sided cushions; the number of arms 

 deciding the number of sides, and stretching out the web in the 

 course of growth into the various shapes as long as it remains 

 attached to them; in slide "Chimborazo Barbados Rd," size 

 .0228. 



2. — Resembles Placolithes radiata. Mikrogeologie PL XXXIV, fig. 5. 

 There are two valves, the upper one plaited like a fan. I have 

 since seen a broken specimen, in which the plaits appear like 

 tubes, hollow, hke the beautiful raised i-ibs on the African Car- 

 dium costatum. Dr. Wallich confirmed the notion of the fan-like 

 plaits, by detecting a crack across one of them. 



3. — A lovely transparent cross, with doubly trifid ends, possibly a 

 sponge spicule; in form most nearly resembling some of the 

 spicules of Hyalonema mirabilis, on Taf. III. of Professor Max 

 Schultze's " Beitrage," &c. 



4. — A transparent Plate, irregularly three-lobed, each lobe bifurcate, 

 in slide "S.P. Barbados." Material from Dr. Kingsley. It 

 appears to be of the nature of those " Hex-radiate plate-spicules " 

 depicted by Dr. Bowerbank as from a " EuplecteUa." 



5. — All explanation of mode of growth of fig. 5 on PI. VII. A Dicty- 

 ochus net ? 



6. — ^A Rhabdolithes, giving a curious example of the way the siliceous 

 outshooting sjiines seem to go wandering on wherever tliey 

 find the least obstruction ; at ' a ' there seems to have been an 

 intended change of course, checked. The extreme fiexibility 

 of these spines might lead to .a supposition whether there might 

 not be combined with the silex in their composition some por- 

 tion of the keratede (or horny matter) stated by Dr. Bowerbank 

 to enter into the formation of even the siliceous coating of the 

 spines. 



7. A double circlet of Acanthodesmia of Miiller. A single circlet is 



given on PI. II, fig. 6. Barbados earth. 



