Hexactinellidas and Lithistidje. 469 



(viz. that enclosed within the body), diminishing' upwards into 

 nothing-, and gradually passing into fully developed spines 

 below. 

 Figs. 7-9 are on the scale of l-12th to l-1800th of an inch. 

 Fig. 10. Fragment of Euplectella cuctimer, to show robust vertical spine 

 of sexradiate spicule in the intervals between the circular open- 

 in^s : a a a, spines ; b b, circular openings ; c, lines of main 

 spicules crossing each other. Diagrammatic. 



Plate XV. 



Sub- skeleton spicules of the Aphrocallistidsa and Aulodictyon. (By " sub- 

 skeleton " are meant the subordinate, not the staple, skeleton-spicules.) 



Fig 1. Scopuline spicule o{ Aphr'ocallistcs Bocagei: a, head; b, shaft; 



c, sexradiate tubercle-inflation of neck ; d, variety in which the 



shaft is somewhat extended beyond the inflation. 

 Fig. 2. Scopidine spicuk of Aphrocallistes beatrix. 

 Fig. 3. Scopuline spicule of Aphrocallistes Bocagei. Occasional variety. 



Like the one in Schmidt's Farrea facunda (I. c). 



N.B. The dotted lines indicate that the parts are microspined. 



Such is the case also with the ends of the shafts, which are here 



represented with smooth lines. 

 Fig. 4. Spino-capitate shaft or spicule of ^^ZofZjrfyoM Woodwardii ; head 



umbonate, many- and small-spined. a, the same, with head 



plano-convex or plain, not umbonate. 

 Fig. 5. Spino-capitate shaft or spicule of Aulodictyon Woodioardii : head 



umbonate, spines few, and microspined on the inner aspect. 



a, the same, with plano-convex head. 



N.B. These two forms appear to be the extremes of the same 

 spicule, which are imited by a variety of transitionary ones. 

 Figs. 1-5 are on the scale of l-12th to l-6000th of an inch. 



Fig, 6. Nail-like skeleton-spicule of Aulodictyon Woodioardii, to show 

 the way in which the four arms are accompanied by the spino- 

 capitate spicules, with which they become included in the 

 vitreous fibre (as shown in the follomng figure) ; a, shaft of 

 nail-like spicule ; b, arms ; c, spino-capitate spicules. Diagram- 

 matic. 



Fig. 7. Portion of the vitrified fibre of Aulodictyon Woodwardii, showing 

 that the spino-capitate spicules are included with the arms of 

 the nail-like spicules in the vitrified skeleton : a, vitrified fibre; 



b, head of spino-capitate spicule, whose shaft is enclosed in the 

 fibre ; c, end of arm of nail-like spicule not enclosed. Scale 

 l-12th to 1-GOOOth of an inch. 



Fig. 8. Fusiform spiniferous spicule, in which all the spines incline the 

 same way. Common to the Aphrocallistida?, Aulodictyon, 

 and Farrea occa (see PI. XVI. fig. 5) ; also to all the Ilexacti- 

 nellidfe possessing the birotulate spicule. 



■Fig. 9. Sexradiate spicule with one arm spined, feather-like. Common 

 to the AphrocallistidjB, Aulodictyon, Sympagella ma; and the 

 Hexactinellidie which possess the birotulate spicule. Scale 

 l-24th to l-(iOOOth of an inch. 



Fig. 10. Sexradiate spicule in which each arm is more or less uniformly 

 beset mth long curved spines. Aphrocallistes beatrix, Euplec- 

 tellidje, and the Hexactinellidie possessing the birotulate 

 spicule. 



The figure represents an unusually perfect form, as regards the 



