300 Mr. J. S. Bowerbank on a new genus of Calcareous Sponge. 



the analogy is still further completed, and the distinction between 

 it and any known crinoidal or echinodermal form is strongly 

 portrayed. If it were a member of either of these two families, 

 it would exhibit on their internal surfaces the same form and 

 number of angles as they do at the external ones ; but this is not 

 the case with the fossil, the inner surface of which presents no 

 appearance whatever of tessellated structure ; but in place of it 

 there is a series of raised lines or ridges running longitudinally, 

 and corresponding in their width and direction with the lines of 

 angular compartments on the exterior. The spaces formed by 

 these parallel lines are again divided, most frequently at right 

 angles, by a number of short raised lines, so that the interior sur- 

 face is covered with numerous, oblong, square, or occasionally 

 triangular sunken areas, presenting exactly the appearance that 

 we should have, supposing that the series of fragile cells, equi- 

 valent to those in the recent Dunstervillia, to be broken away 

 down to their bases, on the inner surface of the outer coat of the 

 sponge. 



The microscopical examination of both the outer and inner sur- 

 faces of the fossil also strongly favours the idea of its spongeous 

 origin. While the surrounding matrix is solid and crystalline, 

 the fossil is exceedingly porous. From the regularity of its struc- 

 ture and the character of its interspaces, it presents strong traces 

 of the original organic arrangement of its parts, and like its re- 

 cent type, the outer surface of the plates is very much more close 

 and even in textiu-e than they are immediately beneath ; and these 

 appearances are not peculiar to the specimen belonging to my 

 friend Dr. Batter sby, but they exist also in an equal degree in a 

 second specimen presented to me by that gentleman. 



EXPLANATION OF PLATE XVII. 



DUNSTERVILLIA ELEGANS. 



Figs. 1 & 2. The sponges of their natural size. 



Fig. 3. The specimen represented by fig. 1. viewed by direct light with a 

 power of 45 linear. 



Fig. 4. A few of the quadrangular and pentangular plates of the same spe- 

 cimen as seen with a power of 94 linear. 



Fig. 5. A view of the terminations of some of the large angulated canala at 

 the inner surface of the sponge with a power of 100 linear. 



Fig. 6. A portion of a section of the sponge at right angles to the natural 

 •surfaces, exhibiting the form and disposition of the angulated ca- 

 nals, seen with a Liebevkuhn and a power of 100 linear. 



Fig. 7. A portion of a section of one of the plates seen by transmitted light 

 with a power of 150 linear. 



Figs. 8& 9. Triradiate spicula magnified 150 linear. 



Fig. 10. One of the simple-double-pointed spicula magnified 150 linear. 



