FOSSIL VASCULAR CRYPTOGAMS 



115 



Radiolarian Rhizopod, may be massulas of Azolla. Sir W. Dawson 

 (Bull. Chicago Acad. Sc., 1886, p. 105) refers organs of fructification ob- 

 tained from the Devonian (=Erian)in Canada and the northern United 

 States and previously described, under the name Sporangites (Daws.), 

 as sporangia of Lycopodiaceae to a genus nearly allied to Salvinia, which 

 he calls Protosalvinia ; but, inasmuch as they are borne on Lepidoden- 

 dron scales, this explanation is inadmissible. Sir W. Dawson believes 

 the megaspores of Rhizocarpeae to be the chief cause of the highly 

 bituminous character of the shales in which these bodies are found. 



FOSSIL SELAGINELLACE^:. 



Remains of arborescent vegetation more or less nearly allied to the 

 typical Selaginellaceae of the present day occur in extraordinary abun- 

 dance in the older fossiliferous strata. Of these the most abundant and 

 best known families are the Lepidodendreae and the Sigillarieae. 



Of the LEPIDODENDREAE the stems are known as Lepidodendron, and 

 the fructification occasionally found in organic connection with the 



FIG. 87. A, B, C, portions of surface of stem of different species of Lepidodendron (natural 

 size) ; Z?, single cushion (magnified). (After Solms-Laubach.) 



branches, as Lepidostrobus. The fructification is distinctly heterosporous ; 

 and although, in a large number of Lepidostrobus cones, microspores only 

 have been detected, this is unquestionably either because the portion con- 

 taining the megaspores has not been preserved, or possibly because the 

 megasporanges and microsporanges may have been distributed in distinct 

 fructifications a degree of differentiation unknown in any existing form. 

 The remains of a large number of species of Lepidodendron occur in 

 the coal measures. They were trees, with stems up to ninety feet in 

 height and two feet in diameter, covered with the diamond-shaped scars 

 of fallen leaves. These scars, together with a portion of the leaf-stalk 

 remaining behind in the form of a cushion, occupied the whole surface 

 of the stem. Wherever the internal structure has been preserved, a 



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