292 



MICROSCOPIC FORMS OF VEGETABLE LIFE. 



boiling nitric acid; the inner cue, siliceous. It is the former 

 which has upon it the peculiar spider's web-like markings ; 

 whilst it is the latter that forms the supporting framework, 

 which bears a very strong resemblance to that of a circular 

 Gothic window. The two can occasionally be separated entire, 

 by first boiling the disks for a considerable time in nitric acid, 

 and then carefully washing them in distilled water. Even with- 

 out such separation, however, the distinctness of the two layers 

 can be made out by focussing for each separately under a l-4th 

 or l-5th in. objective; or by looking at a valve as an opaque 

 object (either by the Parabolic illuminator, or by the Lieberkiihn, 

 or by a side light) with a 4-lOths in. objective, first from one 

 side, and then from the other. 



183. Nearly allied to the preceding in general characters, but 

 dififering in the triangular shape of its valves, is the Trieeratium ; 

 of which striking form a considerable number of species are 

 met with in the Bermuda and other Infusorial earths, while 

 others are inhabitants of the existing ocean and of tidal rivers. 

 The T. favus (Fig. 79), which is one of the largest and most re- 

 gularly marked of any of these, occurs in the mud of the 

 Thames and in various other estuaries on our own coast ; it has 

 been found, also, on the surface of the large sea-shells from 

 various parts of the world, such as those of Hippopus and Halio- 

 tis, before they have been cleaned ; and it presents itself like- 

 wise in the infusorial earth of Petersburg (U. S.) Although the 

 triangular form, when the frustule is looked at sideways, is that 

 which is characteristic of the genus, yet in some of the species 

 there seems a tendency to produce quadrangular and even penta- 

 gonal forms ; these being marked as varieties, by their exact 

 correspondence in sculpture, color, &c., with the normal trian- 

 gular forms.' This departure is extremely remarkable, since it 



breaks down what 

 seems at first to be 

 the most distinctive 

 character of the ge- 

 nus ; and its occur- 

 rence is an indication 

 of the degree of lati- 

 tude which we ought 

 to allow in other cases. 

 The genus Campylo- 

 discus (Fig. 87) is 

 distinguished by its 

 saddle-shaped curva- 

 ture, and by its rib- 

 bed markings, which 

 seem to indicate the presence of canals excavated in or beneath 



" On the genus Trieeratium" in " Quart. Mi- 



Fig. 87. 



Campylodiscus costatus : — a, front view ; Bj side view. 



' See Mr. Brightwell's excellent memoir 

 croso. Journal," vol. i, p. 240. 



