6l2 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



that are precisely alike. It seems probable, then, that we must 

 allow a considerable latitude of variation in these forms before 

 attempting to separate any of them as distinct species. Another 

 very beautiful discoidal diatom, which occurs in guano, and is also 

 found attached to seaweeds from different parts of the w r orld 

 (especially to a species employed by the Japanese in making soup), 

 is the Arachnoidiscus (Plate XII), so named from the resemblance 

 which the beautiful markings on its disc cause it to bear to a 

 spider's web. According to Mr. Shadbolt, 1 who first carefully 

 examined its structure, each valve consists of two layers ; the outer 

 one, a thin flexible horny membrane, indestructible by boiling 

 in nitric acid ; the inner one 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 discs 

 for a considerable time in nitric acid and then carefully washing 

 them in distilled water. Even without such separation, however, 

 the distinctness of the two layers can be made out by focussing for 

 each separately under a J- or \ -inch 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 f^-inch objective, 

 first from one side and then from the other. But it can be seen to 

 very best advantage by the use of apochromatic objectives of 

 suitable power and a suitable diaphragm for dark-ground illumi- 

 nation. 



This family is connected with the succeeding by the small group 

 Eupodiscecv, the members of which agree with the Coscinodiscece in 

 the general character of their discoid frustules, and with the Bid- 

 dulphiece in having areolar processes on their lateral surfaces. In 

 the beautiful Aulacodiscus these areolations 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. 



The members of the next family, Jtiddulphiece, differ greatly in 

 their general form from the preceding, being remarkable for 

 the great development of the lateral valves, which, instead of being 

 nearly flat or discoidal, so as only to present a thin edge in front 

 view, are so convex or inflated as always 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 binary division, but it increases gradually in breadth, 

 until the new frustule is fully formed and is itself undergoing the 

 same duplicative change. In Biddulphia (fig. 445) the frustules 

 have a quadrilateral form, and remain coherent by their alternate 

 angles (which are elongated into tooth-like projections), so as to form 

 a zigzag chain. They are marked externally by ribbings which seem 

 to be indicative of internal costce partially subdividing the cavity. 

 Nearly allied to this is the beautiful genus Isthmia (fig. 457), in 

 1 Trans. Microsc. Soc. 1st series, vol. iii. p. 49. 



