DIATOMACEiE : BIDDULPHIE^ J ISTHMIA. 



301 



Lieberkiikn, or by a side-light) with a 4-10ths inch objective, first 

 from one side, and then from the other.* — This family is connected 

 with the succeeding by the small group of Eujiodiscece, the members 

 of which agree with the Coscinodisceae in the general character of 

 their discoid frustules, and with the Biddulphiese in having tuber- 

 cular processes on their lateral surfaces. In the beautiful Genus 

 Aulacodiscus (Plate i., fig. 5) these tubercles are situated near the 

 margin, and are connected with bands radiating from the centre ; 

 the surface also is frequently inflated in a manner that reminds us 

 of Actinoptychus. These forms are for the most part obtained from 

 Guano. 



230. The members of the next Family Biddidphiece differ greatly 

 in their general form from the preceding ; being remarkable for the 

 great development of the lateral 

 valves, which, instead of being Fig. ls-i. 



nearly flat or discoidal, so as only 

 to present a thin edge in front 

 view, are so convex or inflated as al- 

 ways to enter largely into the front 

 view, causing the central zone to 

 appear like a band between them. 

 This band is very narrow when the 

 new frustules are first produced by 

 self -division (§ 217) ; but it in- 

 creases greatly in breadth until 

 the new frustule is fully formed and 

 is itself undergoing the same dupli- 

 cative change. In Biddulphia (Fig. 

 121) the frustules have a quadri- 

 lateral form, and remain coherent by 

 their alternate angles (which are 

 elongated into tooth-like projec- 

 tions), so as to form a zig-zag chain. 

 They are marked externally by rib- 

 bings which seem to be indicative 

 of internal costce partially subdi- 

 viding the cavity. Nearly allied to 

 this is the beautiful Genus Isthmia 

 (Fig. 134), in 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 Isthmia nervosa. 



of the one next 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 



* These valves afford admirable objects for showing the ' conversion 

 of relief ' inNachet's Stereo-Pseudoscopic Microscope (§ 28). 



