EXPLANATION OF THE PLATES. 



PLATE I. Fig. 1, Tethea Cranium, of the natural size, from a spe- 

 cimen presented to mej by Messrs Edward Forbes and J. Goodsir. 

 Fig. 2, A section of ^the same. Fig. 3, A minute portion of the sur- 

 face magnified. Fig. 4, A fusiform spiculum, and Fig. 5, Three of 

 the tricuspidate spicula of the same. Fig. 6, The oviform bodies mag- 

 nified, and 7, One of them viewed through the microscope after com- 

 pression between two plates of glass. Fig. 8, Three of the cuticular 

 spicula. Fig. 9, The spicula of Tethea Lyncurium. Fig. 10, A 

 minute film of the rind compressed between plates of glass, and highly 

 magnified, to shew the starred spicula. 



PLATE II. Fig. I, Halichondria PALMATA. The figure is about one- 

 half the size of the specimen from which it was drawn. Figs 2, 3, 

 Small specimens of the same species of the natural size ; these are in 

 the collection of Mr Bean of Scarborough. Fig. 4, Two oscula of the 

 natural size, a, as they appear before the superficial network has been 

 removed ; 6, an open osculum with canals opening within its rim. 

 Fig. 5, The spicula. 



PLATE Till. Fig. 1, Halichondria oculata, of nearly the natural 

 size. Fig. 2, The spiculum. Fig. 3, A minute slice of Geodia Zet- 

 LANDICA magnified to shew the structure of the crust and of the in- 

 terior. Fig. 4, A few of the cuticular globules highly magnified. Tlie 

 fibrous fringe about them is produced by the drying of the surround- 

 ing organic mucus. 



PLATE IV. Fig. 1, Haliciiokoria cervicornis, natural size, from 

 a specimen in the collection of the late Mr Templeton. Fig. 2, A 

 small portion magnified. Fig. 3, The pattern of Halichondria ci- 

 nerea as seen through a high magnifier. Fig. 4, Three of its spi- 

 cula. 



PLATE V. Fig. 1 and 2, Varieties of Haliciiondiiia cervicorvis, 

 of the natural size. Fig. 3, The gemmules and simple and starred 

 spicula of Pachymatisma Johnstonia as seen through a high mag- 

 nifier. The preparation given me by .1. S. Bowerbank, Esq. 



