THE DIATOMACE.E OF PHILADELPHIA AND VICINITY 41 



Meridiem 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. 



PI. 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 costse 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-con- 

 stricted form is a passage from the sporangial to the smaller or adult form, or is of no spe- 

 cific 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 char- 

 acterized by less evident costse and is more delicate in general appearance. Some forms 

 are capitate and others are not. Prof. Smith compares the M. intermedium with Peronia 

 erinacea Bre"b. 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 costse 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) FRAGILARIE^E 



Divided into three sections: 



Diatominoe. Valve circular, elliptical to linear, quadrate or cruciform, with transverse 

 costse; without raphe, a pseudoraphe sometimes wanting. 



FragilariinoR. Valve elongate, with small central and terminal elevations, without 

 costse but with transverse punctate strise ; without genuine central nodule. 



Eunotiince. Valve lunate; a raphe sometimes partially formed with terminal nodules 

 near the edges. 



DIATOMIN.E 



Diatoma. Frustules in filaments. Valve linear or elliptical, costate. 

 Plagiogramma. Frustules in fasciae or free. Valve costate. 

 Opephora. Valve costate, with an inner punctate stratum. 



DIATOMA DE CANDOLLE (1805) em. HEIB. (1863) 

 (diatemno, I cut in two) 



Frustules oblong or quadrate, adnate in filaments, attached by alternate angles and 

 finally separating. Valve linear or elliptical, with transverse costae and rows of puncta 

 and a pseudoraphe. 



Chromatophores large granules without definite arrangement. (See PI. 40, Fig. 11.) 



