Meridion constrictum Ralfs, sometimes given as a variety of M. circulare, differs only in the constriction below the apex. The two kinds of frustules are usually found growing together and as the variation is often extremely slight they are here included under the earlier name.
Pl. [10], Figs. 1, 2 and 3.
Fig. 1 represents the constricted form which is the more common. Fig. 3 is a sporangial form.
The sporangial frustules vary in shape and size, some being long and slender, others clavate, but they are all more or less tumid in the middle, with costæ more indefinite than in perfect valves. All gradations occur, one end becoming shorter until the valve has the shape of the variety known as constrictum. It would seem, therefore, that the non-constricted form is a passage from the sporangial to the smaller or adult form, or is of no specific importance. All forms are found living together. The adult frustules are the smaller ones; it is from them that the sporangia are produced.
Meridion intermedium H. L. Smith (Amer. Quart. Mic. Jour., Vol. 1, p. 12) is characterized by less evident costæ and is more delicate in general appearance. Some forms are capitate and others are not. Prof. Smith compares the M. intermedium with Peronia erinacea Bréb. and Arnott which he has named M. erinaceum, hitherto found only in Europe, and points out the relation of the two forms to Licmophora. An examination of the H. L. S. type slides of the two diatoms proves that Peronia has very delicate costæ and a distinct pseudoraphe not noticeable in Meridion. On the slide of Peronia are frustules exactly similar to certain of the sporangial variations of M. circulare.
The fan-like arrangement of Licmophora, the marine form, and the circular chains of Meridion, the fresh-water genus, are similar. Both are stipitate at the beginning of their growth.
(c) FRAGILARIEÆ
Divided into three sections:
Diatominæ.—Valve circular, elliptical to linear, quadrate or cruciform, with transverse costæ; without raphe, a pseudoraphe sometimes wanting.
Fragilariinæ.—Valve elongate, with small central and terminal elevations, without costæ but with transverse punctate striæ; without genuine central nodule.