DIATOMACEJE 



613 



which the frustules have a trapezoidal form owing to the oblique 

 prolongation 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 stipe by which the filament is attached. Like 

 the preceding, this genus is marine, and is found attached to the 

 seaweeds of our own shores. The areolated structure of its surface 

 is very conspicuous both in the valves and in the connecting ' hoop ; ' 

 and this hoop, being silicified, not only connects the two new frus- 

 tules (as at b, fig. 457), 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 truncated appearance (a, c). 



The family Anguliferce, 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 Coscinodiscece 

 and Eupodiscece. Of this family we have 

 a characteristic example in the genus 

 Triceratium, of which striking form a con- 

 siderable 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. T.favus 

 (fig. 442), 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 

 JIaliotis, before they have been cleaned ; 

 and it presents itself likewise in the in- 

 fusorial earth of Petersburg (U.S.A.). 

 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 characteristic of the genus, yet in some of the species 

 there seems a tendency to produce quadrangular &nd even pentagonal 

 forms, these being marked as varieties by their exact correspondence 

 in sculpture, colour, &c., with the normal triangular forms. 1 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 latitude which we ought 

 to allow in other cases. It is difficult, in feet, to distinguish the 

 square forms of Triceratium from those included in the genus 



1 See Mr. Brightwell's excellent memoirs 'On the genus Triceratium' in 

 Quart. Journ. 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 

 Trans. Microsc. Soc. n.s. vol. ix. 1861, pp. 43, 69. 



FIG. 457. Isthmia nervosa. 



