Z A MITES. 



77 



gives a diagnosis of Zamites in which the veins in the pinnae are 

 described as equal and parallel, and dichotomously converging 

 towards the apex. He thus includes such forms as correspond 

 to Brauu's genus Podozamites . In Zittel's Sandluch l we find a 

 closer approximation to the definition of Zamites as generally 

 accepted : the pinnae are described as attached to the upper surface 

 of the rachis, and possessing a basal callosity, rounded or some- 

 what contracted at the base ; the median veins parallel, and the 

 outer veins diverging towards the apex of the pinna. 



Solms-Laubach 2 notes the occurrence in the Zamita of pinnae 

 which articulate with and separate from the frond axis, pointed 

 at the apex, abruptly rounded at the base, and attached obliquely 

 to the rachis, which they overlap and cover. Solrns pertinently 

 remarks that isolated segments of species of Zamites may easily 

 be mistaken for portions of other fossil genera. Before attempting 

 to modify in any way the definitions of Zamites, it may be well 

 to consider some of those special features which have been 

 referred to by several writers, and to see how far such details 

 of frond structure are likely to serve as trustworthy guides. As 

 regards the basal callosity usually included in definitions of this 

 genus, we must acknowledge the great difficulty to be experi- 

 enced in deciding definitely as to its existence in many fossil 

 leaves. In the process of fossilization the pinnae of a cycadean 

 frond are often flattened down against the rock, and closely 

 adpressed to the surface of the rachis, and thus there may be 

 produced transverse wrinklings just above the point of attachment 

 of the segments; such wrinklings may easily suggest in a fossil 

 specimen the original existence of a basal callosity. The venation 

 may prove useful in determining certain species, but it is not 

 very often that fossil specimens are sufficiently well preserved to 

 admit of a complete diagnosis of the venation character. If, for 

 example, we had neither the basal nor the apical portion of a 

 pinna, it would be practically impossible to discriminate between 

 some of the long narrow Otoxamites pinnse, and those of Zamites 

 and other genera. 



Nathorst's recently proposed genus Zamiophyllum, to which 

 reference has already been made, was founded partly to include 



1 Zittel (A.), p. 218. 

 3 Fossil Botany, p. 88. 



