THE PAST HISTORY OF MONOCOTYLEDONS 171 



Stems, leaves, or rhizomes, might be referred to Butomus. It is a 

 marsh jjlant and is found at the present day in Europe, N.W. Asia, 

 N.W. India, having a similar distribution to other Helobiese. 



There are two fossil species which have some claim to be in- 

 cluded in Butomus. These are the Tertiary Butomus acheronticu?n, 

 Heer, found at G^ningen, and B. heerii, Ett., found at liilin. 



No fossils can be assigned to Elisma or Damasofiium, the former 

 is now confined to N.W. Europe, and the latter to Europe, Cali- 

 fornia, Australia. They are not found in Preglacial or Glacial beds. 



viii. HydrocharidecB. 



The Hydrocharideae are nearly all aquatic plants with aerial float- 

 ing or submerged leaves. Their distribution is confined mainly to 

 the Tropics, but some species occur in temperate regions, and some 

 inhabit tropical seas. There are about forty living forms. The 

 leaves are orbicular in Nydrocharis, ensiform in Stratiotes and linear 

 in Elodea, to take three dominant British types of the present day. 

 Fossil forms have been referred to Stratiotes, Hydrocharis, Valtisneria, 

 Ottelia. The genus Vailisneria is distributed at the present day over 

 both hemispheres. It contains two species, both of which are fresh- 

 water species. V. spiralis is confined to Australia. It is remark- 

 able for the curious manner in which fertilisation is effected. 

 The male flowers are short and becoming detached float on the 

 surface of the water, but the female is borne on a long spirally 

 coiled stem which rises to the surface and allows the flower to float, 

 and in time come in contact wnth the detached male flowers. 



Vailisneria bromelicefolia, Sap., Eocene, Aix, has leaves of a 

 general character resembling those of the recent genus. 



The fossils based on the leaf-fragments from the Jurassic of 

 Siberia, and called Vallisnerites Jurassicus by Heer, must be regarded 

 as open to question. 



The flower described by Heer as Stratiotites naiadum, from the 

 Tertiary of CEningen, is doubtless referable to the genus Stratiotes. 



In this genus the flowers are also submerged before flowering 

 takes place, but they float upon the surface at the period when 

 fertilisation is to be accomplished, and the fruit when formed again 

 sinks. 



The general inflorescence and perianth is like that of Hydro- 

 charts, but the female flower is sessile and is seated on a 2-spathed 

 peduncle. The distribution of Stratiotes is limited to Europe and 

 Siberia, but it is not found in Greece. 



The British species is found in ponds and ditches on the east 

 coast, and has been met with in Preglacial and Interglacial beds in 

 Britain. 



Hydrocharis is a floating plant with entire orbicular leaves, H. 

 fnorsusratice, L., having reniform orbicular leaves. Heer described 

 a species, H. orbiculata, which resembles the living species sufficiently 

 in form. The fossil figured by Weber from the Tertiary of Bonn, 



