Freshwater Limestone in Forfarshire, Sgc. 91 



alluded to by M. Lemarij do not appear in C. vulgaris, nor in any of the Charae 

 mentioned in this memoir. In some of these, as in C. vulgaris, septa occur at 

 the points where the leaves or small branches are set on ; or even indepen- 

 dently of these points, in C.Jlcxilis* (see PI. XII. fig. 4 — 6) ; but in these cases 

 they are distant from each other. 



Seed-vessels of Charts . — The pericarpia or seed-vessels of Charae consist 

 of a membranous nut covered by an intcj^ument. (See PI. XII. fig. 1./) The 

 nut is spirally striated or ribbed. In some specimens of C. hispida, from the 

 Bakie, the nut inclosed within a fossilized integument, admitted of the spiral 

 parts being separated, precisely like the valves of a ripe integument. (See 

 PL XII. fig. l.f) It is of consequence to observe that in this part of their 

 structure, the nut and the integument do not appear to differ ; for the nut of 

 the ancient fossil gyrogonites being sometimes found without the integument, 

 were we not aware of this structure, might be supposed to belong to a distinct 

 species. 



The integument is composed of 5 spiral valves. These valves are of a 

 quadrangular form (see section of one, PI. XII. fig. I. i.) : they are attached 

 by their inner edges to the striae on the nut, which is well exemplified in sec- 

 tion of C. Medicaginula from the Isle of Wight. (See PI. XIII. fig. 2. b.) The 

 valves of the living plant appear, under the miscroscope, to contain within a 

 thin membrane a transparent brittle substance, perhaps carbonate of lime. An 

 outline drawing of a species of Helicteres from the West Indies, with which 

 Mr. Brown has favoured me, has been introduced, as its shape offers a strik- 

 ing subject of comparison with the integument and nut of the pericarpium of 

 the Chara. (See PI. XII. fig. 5.) The analogous form is in both instances 

 produced by the twisting together of 5 spiral tubes ; although the analogy does 

 not hold when the parts of the plants are compared, of which these tubes are 

 constituted : for in the Helicteres they consist of 5 spiral capsules, but in the 

 Chara, of 5 spiral valves, forming together the integument of a single capsule. 

 It should also be observed that the capsules in the Helicteres wind round in 

 a contrary direction to the valves of the Chara. 



Stems of Chares. — The stems of Charge may in general be described as 

 composed of a large tube, surrounded by smaller tubes. (See PI. XII. fig.l. m. 

 fig. 2. g, h. fig. 3. g-. PI. XIII. fig. 6.) In some, however, there is only one 

 single tube. The smaller tubes are irregular in number. In specimens of 

 C. hispida, for instance, the numbers 10, II, 12, and 13, were observed. 



Gyrogonite of the Bakie. — It may be convenient to retain the term Gyrogo- 



* See Note, p. 93. 



n2 



