AEACKN^ODISCUS. lo5 



Here then is a box formed of pure transparent flint- 

 glass, very thin and delicate, and very brittle. The 

 valves are marked with minute dots, which appear to be 

 either knobs or pits; or with lines, either depressed or 

 raised. In the beautiful Arachnodiscus, both of these 

 modes of sculpturing are present. Each valve is marked 

 with a number of most delicate lines, which radiate from 

 a central circle of dots to the circumference ; these radii 

 are connected by a multitude of cross lines, bearing the 

 closest resemblance to the elegant webs spun by our 

 common geometric spiders, whence the name given to the 

 genus ; while in the spaces marked out by these reticula- 

 tions there are rows of minute round dots. Altogether, the 

 effect of this complex pattern of sculpture is most charming, 

 and is heightened by the brilliant translucent material in 

 which it is wrought, which, as has already been observed, 

 is like the purest glass. 



During life there is, in every individual, a small round 

 body in the centre of the enclosed cavity, called the nucleus, 

 and this is surrounded by irregular masses of yellowish 

 substance, called the endochrome, the nature of which is 

 not very clearly ascertained. The single specimen, in- 

 cluding the two valves and the hoop, with their contents, 

 is called a,frustule. 



The manner in which these l^eautiful, but most minute 

 atoms increase, is highly curious. The piU-box-like frus- 

 tale becomes deeper by the widening of the hoop, thus 

 pushing the valves further from each other ; then across 

 the middle two membranes form, which, by and by, from 



