1872.] DR. J. S. BOWERBANK ON THE SPONGIADjE. 127 



tomical proportions of its essential organs never vary. The fusiformi- 

 spinulate skeleton-spicula vary somewhat in size ; and while some of 

 the spinulate bases are globular, others are more or less clavate. 

 An average-sized spiculum measured ■£$ inch in length, and greatest 

 diameter 2 1 1 4 :i inch. 



The history of this sponge has more than usual interest, as it 

 tends to reconcile and explain difficulties in our knowledge of the 

 ancient sponges, many of which in the lower beds of the Portland 

 Oolite have evidently been of enormous size. Among the debris of 

 the cliffs of Portland not very far on the shore, beyond the little 

 village of Fortune's Well at the head of the great Chesil bank, I recol- 

 lect seeing one mass more than a yard square that had evidently 

 been one large sponge, with numerous anastomosing branches, each 4 

 or 5 inches in diameter, of silicified matter, with the large interstices 

 between the ramifications filled up with pure Portland Oolite. And 

 during the excavation of the great dry cutting that encloses and 

 protects the fortifications I observed that they had cut through nu- 

 merous masses of flint, many of which were from 7 to 10 feet in 

 length and 2 or 2| feet thick at the middle, gradually thinning 

 away towards their margins. I examined fragments of many of these, 

 and found them quite as full of the remains of sponge-tissues, Poly- 

 thalamia, fragments of minute corals, and other extraneous matters 

 as the smaller flints of the same oolitic beds, the greensand, and the 

 chalk flints. From the great size of Hymeniacidon pulvinatus 

 among living sponges, we may well imagine that the fossil siliceous 

 masses in the lower beds of the Portland Oolite may have been in 

 their day what Hymeniacidon pulvinatus at Calibert Quay, near 

 Honduras, now is in those seas. 



It has been said by eminent naturalists that there appears to be 

 no known limit to the increase in size of the large crocodilian reptiles ; 

 and the same may well be said of many of the sponges. Their 

 organic structures (young or old, large or small) never vary to any 

 appreciable extent in size or form ; and the vast difference in those 

 respects that frequently exists in two individuals of the same species 

 is due only to a multiplication of their internal organs. I have other 

 specimens of H. pulvinatus in my possession — one 12 or 14 inches in 

 diameter, 8 or 9 inches in height, and a smaller one, not exceeding 

 6 inches in diameter by about 4 inches in height. All of the speci- 

 mens are of the same massive cushion-like form. 



DESCRIPTION OF THE PLATES. 



Plate V. 



Tefhea murkata, Bowerbank. 



Fig. 1 represents the type specimen of the species, of the natural size, from Ham- 



merfest. 

 Fig. 2. Half of one of the large fusiforrni-acerate skeleton-spicula, magnified 36 



linear. 

 Fig. 3. One of the simple attenuato-expando-ternate connecting and defensive 



spicula, magnified 36 linear. 



