ME. S. O. RIDLEY ON THE GENUS DIRRHOPALUM. 481 



more powerful uniting agent than mere sarcode : such a material 

 is sometimes to be distinctly seen, and when it is found to polarize 

 light may perhaps be still held to be keratose ; where it does not, 

 it may be termed pseudokeratose *. The tough, dark, keratose-like 

 substance of the stem of D. manaarense exhibits decided polarizing 

 effects, but the similar matter in D. novizelanicum does not. 

 The following must be added to the genus : — 

 4. Dirrhopalttm CORIACEUM. (Plate XXIX. figs. 3-7.) 

 Isodictya coriacea, Bowerbank, Mon. Brit. Spong. iii. p. 228, pi. lxxvi. 



figs. 7-12. 

 It was obtained in Strangford Lough, Ireland. The original 

 description is misleading, so I give the following supplementary 

 account of the structure, made from Dr. Bowerbank's own pre- 

 parations. 



Skeleton. — Primary lines composed of (1) long, smooth acuate 

 and (2) shorter spined acuates, the latter chiefly echinating the 

 fibre by the lateral outward projection of their points at a very 

 acute angle to it. Secondary lines, one spicule in length, com- 

 posed of from one to three dumbbell-shaped spicules (3) at 

 right angles to the primaries. Dermal sarcode granular, very 

 dark ; subjacent sarcode dark ; a yellowish material unites the 

 primary and secondary lines, but it does not polarize light. 



Skeleton- spicules of three kinds : — (1) Large smooth acuates, 

 slightly inflated, constricted above base, thickest immediately 

 above this constriction ; size variable, viz. -317 to "4434 by '01268 

 to *014 millim. (2) Smaller acuates thickly spined at base, very 

 sparsely over the whole of the shaft ; size '158 by '0079 millim. 

 (3) Cylindrical dumbbell- shaped spicule ; ends slightly inflated 

 and well spined ; the shaft less strongly spined (a converse im- 

 pression is conveyed by fig. 12 of Mon. Brit. Spong. iii. pi. lxxiv.); 

 size -117 by -0079 millim. 



Flesh-spicules of two kinds : — (1) Tricurvate acerate, much 

 more slender in proportion to its length than as given in Dr. 

 Bowerbank's figure (I. c. fig. 9), and the ends carry a few minute 



* I have experimented with the polariscope in order to discover, if possible, 

 some real difference in optical properties between ordinary sarcode and keratose 

 in the living matter of Sponges. The results are remarkable : thus the horny 

 matter, mounted in balsam, of Tuba (a Chalinid), of Rhizochalina oleracea, 

 Hircinia lingua, and Euspongia virgultosa polarizes light, while that of Chalina 

 finitima does not ; the sarcode was never found to polarize. Quekett (' Practical 

 Treatise on the use of the Microscope,' edit. i. p. 448) ranks sponge-fibre with 

 hoof, horn, and other ceratinous bodies as having this property of polarizing light. 



