366 W. J. SOLLAS ON" THE STRTJCTI7EE AIS^D 



the walls of opposite sides close together ; or it may have been an 

 incrustiug flat film, and the two laj^ers may indicate the snccessive 

 growth of two different individuals on the same spot ; or, finally^ 

 the two layers may represent the outer and inner surface of one and 

 the same sponge-wall, which in this case must have been of some 

 considerable thickness. 



Form of S])onge. — Of this also we are entirely ignorant. It may 

 have been flat and incrusting; or, on the other hand, it may have been 

 sacciform and anchored in the slimy ooze of the sea-bottom by a 

 tuft of anchoring spicules, of which Dr. Hicks's Protospongia major, 

 may be the imperfectly preserved remains. 



Mineral State of the Sjjicules. — Without doubt the spicules were 

 originally siliceous; they consist now, however, of iron-pyrites, which, 

 by superficial oxidation, has led to the coating of the spicules with a 

 thin red layer of iron-rust. 



Taxonomic Position of the Sponge. — The position of the sponge 

 with regard to others is by these observations definitely fixed. It 

 consists of cruciform spicules of various sizes disposed to form a 

 regular square meshwork, the rays of the smaller spicules underly- 

 ing at their terminations the rays of the larger ones. Such spicules- 

 so disposed are to be met with among the Hexactinellid Sponges 

 alone ; the absence of one (or possibly two) of the rays which should 

 be present to render the spicules literally hexactinellid is of no im- 

 portance ; in several characteristic forms of recent Hexactinellid^ 

 similar spicules are common, along with others of the genuine sex- 

 radiate type. The suppression of one, two, three, or even four 

 arms of the hexactinellid spicule may easily take place without af- 

 fecting its undoubted Hexactinellid character. 



Again, the spicules of Protospongia are separate ; they are not 

 united either by envelopment in a common coating or by ankylosis : 

 and hence they must clearly be assigned to the Lyssakina of Zittel, 

 a group nearly equivalent to Carter's division of the Sarco-hex- 

 actinellidae. 



The nearer relations of Protospongia cannot at present be defined. 

 Its chief differences from other Lyssakine Hexactinellids depend on 

 negative characters, and consequently may very possibly be but the 

 expression of our present imperfect knowledge of its original nature. 

 Por the present we must rest content with the knowledge of the 

 fact that it is a true Lvssakine Hexactinellid. 



DiSCFSSION. 



Prof. DimcAN expressed his pleasure at hearing so good a descrip- 

 tion of an ancient sponge. We were much indebted to Bowerbank, 

 Carter, Zittel, and Mr. Sollas for their work among the sponges. He 

 had himself obtained from a coral dredged from about 500 fathoms 

 in the Atlantic a sponge which presented some analogies to Proto- 

 spongia. He would be glad to know if Mr. SoUas thought Proto- 

 spongia part of the dermal structure of a large sponge. 



