OF THE DESMIDIE.E. 3 



the point of separation into two valves (II. 1, 2, 9). Each valve again is 

 occasionally subdivided by another line (II. 6, 15). These lines may be 

 single or double, and in the case of the middle suture their number may be 

 more multiplied, as in Closterimn lineatum and C. Rcdfsii. The median 

 sutiu^al line is e\ddent in other genera, e. g. in Hyalotheca, Cosmarhim, and 

 Euastrum (II. 35). In several it takes on a further development, and 

 becomes an elevated ridge or band, appearing, in a front \iew, as a double 

 line, terminating on each margin in a dentation. Instances occur in Docidium 

 and in DidymoiJrmm (II. 9, 39). Such double lines are also sometimes met 

 with on each side the median suture, and at others, among the concatenate 

 forms, at the junction-siu'faces of connected fronds. 



That the dots or pimcta on the surface of the frustules are commonly small 

 elevations has already been stated; a further development of such into 

 papillae or minute spines crowned by a globular apex is seen in Micrasterias 

 jpapUlifera ; whilst in many Cosmaria and Staurastra, the edge or the entire 

 surface is bedecked by fine hair-like spines or by obtuse ones, looking on the 

 margin like crenations (I. 1, 2, 3). When short and stout, many elevated 

 processes of the surface are called tubercles (II. 16, 17) ; when long and 

 tapering, they constitute spines, and in this form may be either straight or 

 curved : such are especially produced from the angles of the fronds, as in 

 Arthrodesmus (II. 18, 28). Among the Staurastra, illustrations of forked 

 spines (II. 3, 7) are found ; whilst among sporangia of many species, spinous 

 processes, besides tubercles and other appendages, are highly develoi)ed (II. 

 22, 25, 34) and attain their most complex conditions. 



The modification of surface in several genera seems due, not to mere simple 

 appendages, but to positive expansions of the limiting membrane itself into 

 thick processes, which in their turn usually end in spines ; instances occur 

 in Xanthidium and Staurastrum (I. 27, 28 ; II. 3, 7, 20, 25). Generally 

 these large productions from the surface occupy constant and definite positions, 

 such as the extremities, the rounded angles of the fronds, or a margin, and 

 are rarely indifferently placed. A general distribution over the surface is 

 rather characteristic of Xanthidimn (I. 27, 28). In Euastrum the surface is 

 thrown into very broad round swellings, hence caUed inflations ; such may be 

 presumed to be constant in number and position (I. 24, II. 30, the empty 

 divided fronds). 



The margin of the more flattened, and the extremities of the elongated, spe- 

 cies furnish important specific and generic characters. Micrasterias has its 

 margin deeply incised into lobes (1.18, 20, 21, 22), which, with reference to the 

 centre of the frond, have a radiating arrangement, and are themselves incised 

 or inciso- dentate. The fronds of Euastrum are more or less deeply sinuated 

 (I. 23, 24, 26 ; II. 10), and the intermediate lobes produced vary both in 

 dimensions and outline. Where the lobes on the margin of fronds are small 

 and little prominent, they constitute crenations and dentations which may 

 occur singly or in paii'S ; in the latter case, the margin so modified is said to 

 be bidentate or bicrenate (I. 1, 2, 3 ; II. 31, 26, 37). For example, some 

 fronds of Euastrum binatum are bicrenate on the sides, and those of Didy- 

 moprium at the angles of the filaments (II. 39), whilst bidentate frustules 

 are seen in Desmidium (II. 37), and in Hyalotheca mucosa. It has been 

 before remarked that when the surface is covered by tubercular eminences 

 or conical granules, a dentate outline is produced ; instances of this occur in 

 EimMrum verrucosum and in several Cosmaria. Another variety of margin 

 exists, known by the term undulated or wavy, where its elevations and de- 

 pressions are comparatively shallow. Lastly, the , general concavity or the 

 convexity of the margin furnishes other specific characteristics. 



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