120 MISS M. E. J. CHANDLER OK THE [vol. lixix, 



The tegmen is thin, and is attached at the base to the testa by 

 a thread which passes into the rnicropyle ; at its apex a black hat- 

 shaped structure (the 'caruncula' or 'hiitchen' of German writers) 

 marks its attachment to the chalaza (fig. 2, p. 119). The cells of 

 the tegmen are elongate, with beaded walls (PI. YI, fig. 21). In 

 recent seeds the embryo lies within the tegmen ; but, as it is 

 absent in the fossils, where the seed-cavity is occupied by the 

 empty tegmen only, it does not concern us here. 



The length of fully developed recent seeds is from 9 to- 

 7*25 mm., and the breadth about 2'5 mm. 



IV. The Fossil Species. 



The fossil seeds all conform to the general structure described 

 above, although they show modifications in detail, giving rise to a 

 graded series of species so closely linked that, in the absence of all 

 parts of the plant but the seeds, it is illogical to separate any one 

 species as a distinct genus. Attention is drawn to this point, 

 because attempts have been made among German palseobotanists- 

 to retain the Tertiary species as Folliculites (44, 60, 70, 72, 73). 



As the result of recent investigations, nine species are 

 recognized : — 



9. Stratiotes aloides Linn. : Pleistocene and Pregiacial. 



8. S. intermedins Hartz : Pregiacial. 



7. 8. tuberculatus E. M. Eeid : Lower Pliocene and Upper Miocene. 



6. 8. Tialten7iordheimensis Zenker : Lower Miocene. 



5. 8. thalictroides Brongniart : Upper Oligocene. 



4. 8. acuticostatus, sp. nov. : Hamstead Beds, Middle Oligocene. 



3. S. websteri Zinndorf (? Brongniart) : Hamstead Beds, Middle Oligocene. 



2. S. neglectus, sp. nov. : Bembridge Beds, Middle Oligocene. 



1. 8. headonensis, sp. nov. : Lower Headon Beds, Upper Eocene. 



Thus a series is known stretching far back into the Tertiary, 

 some forms being close in time, others separated by gaps which 

 further search may ultimately help to fill. 



V. Evolutionary Changes, and the Eelationship 



BETWEEN THE DIFFERENT SPECIES. 



In studying evolutionary changes, it is logical to pursue them in 

 the order in which they have taken place in Nature ; therefore, the 

 changes in the genus will be followed from the oldest to the newest 

 species. 



The form of the seed. — Eocene seeds are small, short, and 

 broad ; Oligocene, Miocene, and early Pliocene seeds are (on the 

 whole) sturdier and longer ; Pregiacial, Pleistocene, and recent 

 seeds are long and narrow (see PL V, figs. 1 to 23). 



The collar and neck. — In Eocene, Oligocene, and Miocene 

 species the neck is well-defined, hence the collar is conspicuous ; 

 Pregiacial, Pleistocene, and recent examples have an ill-defined 



