ABSTRACT OF THE INFUSIONSTHIERCHEN OF EIIRENBERG. 115 



It frequently occurs in the Echinitcs which are filled with flint, that 

 portions of the shell have been replaced or infiltrated with silex. In 

 all these cases that I have seen, the silex presents a stalactitical or chal- 

 cedonic form, and never exhibits the spongeous texture. Very fre- 

 quently, however, thin laminae of spongeous texture are found to have 

 been built between the plates of the Echinite, where they have happened 

 not to have been quite in contact ; and in these laminae the tubuli are 

 as beautifully distinct as in the most favourable portions of the mass of 

 the cast. If the surface of the cast be microscopically examined, we 

 shall frequently observe that the flint has not been in such a state of 

 contact with the shell as a cast from a fluid material would be supposed 

 to present ; for although the boundary of each plate is well marked, the 

 areas of their impressions exhibit such a view of the tubuli of the 

 sponge as we might naturally expect to find where numerous minute 

 tubes have been built against a flat or slightly concave surface. Some- 

 times the sponge has grown round the interior of the shell, and has 

 left a hollow near its centre ; and occasionally the sponge appears closely 

 approximating, yet not quite adhering, to the inner surface of the Echi- 

 nite. In these cases, a thin film of chalcedony is frequently spread 

 over the organized surface of the sponge, which, in a specimen in my 

 possession, is in several places to be seen through small breaks in the 

 film. 



(To be continued.} 



XXVI. AN ABSTRACT OF THE " INFUSIONSTHIERCHEN" OF 



EHRENBERG. No. 2. 



By W. Hughes Wiltshire, M.D.,M.B.S., Physician to the 

 Fore Street Dispensary, #c. 



THE Infusoria are divided by Ehrenberg into two great classes, Poly- 

 gastrica and Rotatoria. The Polygastrica are characterised by having 

 their intestinal canal provided with numerous digestive sacs, or stomachs, 

 and by their non-possession of a true rotatory apparatus. The Rotatoria 

 are distinguished by their intestinal canal being simple, and by their pos- 

 session of peculiar ciliated organs, by means of which they perform 

 certain rotatory movements. The following are the characters of the 

 Class Polygastrica, to which is added a Synoptical Table of the Fa- 

 milies. 



i2 



