302 DIATOMACE.E : ANGULIFERE^E J TRICERATIUM. 



the Algce of our own shores. The areolated structure of its surface 

 is very conspicuous (Fig. 118) both in the valves and in the connecting 

 ' hoop ;' and this hoop, being silicified, not only connects the two 

 new frustules (as at b, Fig. 134), until they have separated from 

 each other, but, after such separation, remains for a time round one 

 of the frustules, so as to give it a truncate dappearance (a, c). 



231. The Family A ngidiferece, distinguished by the angular form 

 of its valves in their lateral aspect, is in many respects closely allied 

 to the preceding ; but in the comparative flattening of their valves 

 its members more resemble the Coscinodisceas and Eupodiscese. Of 

 this family we have a characteristic example in the Genus Tricer- 

 atium'; 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. 119), which is one of the largest and most regularly- 

 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 large Sea-Shells from various parts of the world, 

 such as those of Hippopus and Haliotis, before they have been 

 cleaned ; and it presents itself likewise in the Infusorial earth of 

 Petersburg (U.S.). The projections at the angles which are shown 

 in that species are prolonged in some other species into ' horns,' 

 whilst in others, again, they are mere tubercular elevations. Al- 

 though the triangular form of the frustule when 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 

 pentagonal forms ; these being marked as varieties by their exact 

 correspondence in sculpture, colour, &c, with the normal triangular 

 forms. * This departure is extremely remarkable, since it breaks 

 down what seems at first to be the most distinctive character of the 

 genus ; and its occurrence is an indication of the degree of lati- 

 tude which we ought to allow in other cases. It is difficult, in 

 fact, to distinguish the square forms of Triceratium from those 

 included in the Genus A mphitetras, which is chiefly characterized 

 by the cubiform shape of its frustules. In the latter the frustules 

 cohere at their angles so as to form zig-zag filaments, whilst in the 

 former the frustules are usually free, though they have occasionally 

 been found catenated. — Another group that seems allied to the 

 Biddulphiea? is the curious assemblage of forms brought together 

 in the Family Clmtocerew, some of the filamentous types of which 

 seem also allied to the Melo&ireaz. The peculiar distinction of this 

 group consists in the presence of tubular 'awns,' frequently pro- 

 ceeding from the connecting hoop, sometimes spinous and serrated, 



* See Mr. Brightwell's excellent memoirs ' On the genus Triceratium,'' 

 in " Quart. Journ. of Microsc. Science," Vol. i. (1853), p. 245, Vol. iv. 

 (1856), p. 272, Vol. vi. (1858), p. 153 ; also Wallich in the same journal, 

 Vol. vi. (1858), p. 242; and Greville in "Transact, of Microsc. Soc," N.S., 

 Vol. ix. (1861), pp. 43, 69. 



