part 2] GEOLOGICAL HISTORY OF THE GENUS STKATIOTES. 121 



neck and small collar ; consequently, body and collar are less clearly 

 distinguished one from the other, and the hooked form of the 

 seed, due to the lateral projection of the collar, is less marked (see 

 PI. V, figs. 1 to 23). 



The keel has passed through three stages: (a) in the Eocene 

 species it stops short of the base on the dorsal side, does not merge 

 into the collar, but terminates against the neck or the top of the 

 collar (PI. V, figs. 1 & 2). (b) In Oligocene and Lower Miocene 

 species it still ends on the dorsal side, but extends to the base of 

 it, and gradually merges into the collar (PL V, figs. 4 to G, 7, 

 10 to 15). (c) In Upper Miocene to recent species it is continued 

 round the base of the seed, below the collar, and terminates on 

 the ventral side (PL V, figs. 16 to 23). 



The testa. — In older forms the testa is thick and woody, the 

 external ornamentation rugged and pronounced. In the Eocene 

 species especially, but also in some Oligocene species, the longi- 

 tudinal rows of tubercles of the external surface are so arranged 

 as to give a ribbed appearance to the seeds (PL V, figs. 1 to 12). 

 In Miocene and Pliocene species the elongate tubercles still tend 

 to be arranged in longitudinal rows, and are still abundant (PL V, 

 figs. 15 to 17). In the Preglacial Stratiotes intermedins, tubercles 

 are very distinct, but sparsely scattered (PL V, figs. 18 & 19). 

 In Pleistocene S. aloides, tubercles may or may not be present 

 (PL V, figs. 20 to 22) : if present, they are ill-defined and few. 

 In recent S. aloides, tubercles are absent ; but occasionally the 

 surface is obscurely angulate (PL V, fig. 23). 



The micropyle. — In Headon and Bembridge seeds, basal or 

 sub-basal, only slightly oblique (PL V, figs. 24-28) ; in the 

 Hamstead species, S. acuticostatus, sub-basal and oblique (PL V, 

 figs. 29 & 30) ; in Lower Miocene seeds, sub-basal and very 

 oblique (PL VI, figs. 9-11) ; in Upper Miocene to recent species 

 basiventral and very oblique, horizontal or even recurved (PL VI, 

 figs. 12-15, & 17-20). 



The raphe. — -In Upper Eocene species, usually marginal from 

 base to apex, passing directly to the seed-cavity (PL V, figs. 25 & 26) 

 at the apex ; in Middle Oligocene species, marginal from the base 

 to the middle of the dorsal side, thence diagonal to the apex, the 

 diagonal portion being therefore abrupt and short. Two minor 

 modifications occur in the Oligocene; in S. neglectus (Bembridge) 

 the diagonal part is curved, and divides the dorsal wall either 

 equally, or so that the part external to the diagonal is rather 

 narrower than the part internal to it; in the closely allied 

 S. acuticostatus (Hamstead) the diagonal part of the raphe is 

 less curved, and so divides the dorsal wall that the width of wall 

 outside is equal to, or more commonly greater than, the width 

 inside (see PL V, figs. 27 & 28 with figs. 29 & 30). 



In Lower Miocene to recent species the raphe runs diagonally 

 from the basal hilum to the apical chalaza. S. Jcaltennordheim- 

 ensis shows the first occurrence of this long diagonal type (PL VI, 



