MICROSCOPIC FORMS OF VEGETABLE LIFE. 



295 



longation of the valves; the lower angle of each frustule is coherent to 

 the middle of the next one beneath, and from the basal frustule proceeds 

 a stipes by which the filament is attached. Like the preceding, this 

 genus is marine, and is found attached to the AlgcB of our own shores. 

 Theareolated structure of its surface (Fig. 163) is very conspicuous both 

 in the valves and in the connecting ' hoop;' and this hoop, being silici- 

 fied, not only connects the two new frustules 

 (as at b, Fig. 181), until they have separated 

 from each other, but, after such separation, re- 

 mains for a time round one of the frustules, so as 

 to give it a truncated appearance (, c). 



293. The Family Anguliferece, 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 

 Coscinodiscem and Eupodiscece. Of this family 

 we have a characteristic example in the genus 

 Triceratium; of which striking form a consider- 

 able number of species are met with in the Ber- 

 muda and other Infusorial earths, while others 

 are inhabitants of the existing ocean and of tidal 

 rivers. The T. favus (Fig. 164), which is one 

 of the largest and most regularly-marked on 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 flaliotis, before they 

 have been cleaned; and it presents itself like- 

 wise 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. Although the triangular 

 form of the frustule, when looked at sideways, is that which is character- 

 istic of the genus, yet in some of the species thepe seems a tendency to 

 produce quadrangular and even pentagonal forms; these being marked 

 as varieties by their exact correspondence in sculpture, color, etc., with 

 the normal triangular forms. 1 This departure is extremely remarkable, 

 since it breaks down what seems at first to be the most distinctive char- 

 acter of the genus; and its occurrence is an indication of the degree of 

 latitude 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 Amphitetras, which is chiefly characterized by the cubiform 

 shape of its frustules. In the latter, the frustules cohere at their angles 

 so as to form zigzag filaments, whilst in the former the frustules are usu- 

 ally free, though they have occasionally been found catenated. Another 

 group that seems allied to the BiddulphiecB is the curious assemblage of 

 forms brought together in the Family CJmtocerecB, some of the filament- 

 ous types of which seem also allied to the Melosirece. The peculiar dis- 



'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. iv. (1858), p. 242; 

 and Greville in "Transact, of Microsc. Soc.," N. S., Vol. ix. (1861), pp. 43, 69. 



Isthmia nervosa. 



