228 DIPTEROID FERNS [CH. 



a type common among the Simplices, to a type characteristic of the highest 

 Leptosporangiate Ferns. They all belong to the Superficial Series, and in all 

 of them (excepting the peculiar case oi Matonid) indusial protection is absent, 

 while in none of them is there a gradate sequence of the sporangia. This 

 general position may be presented in converse by saying that all the steps 

 in advance which their sori show have been carried out without any suggestion 

 of a marginal origin, and without reference to those indusial flaps which so 

 often characterise marginal sori. Ultimately an Acrostichoid state is reached 

 in Cheiropleuria, Christopteris and Hyvienolepis, comparable with that of 

 Acrostichunt aureum itself, though comparison clearly shows that the steps 

 leading to the latter have been along quite distinct phyletic lines. There is 

 good reason to believe that the progression here indicated has pursued its 

 own course in these Superficial Ferns or their derivatives, quite independently 

 of any similar progressions that may have been executed among Marginal 

 Ferns or their derivatives. For the Gleichenioid type existed certainly in 

 early Mesozoic time, and it probably had a Palaeozoic origin in some such 

 type as Oligocarpia. In fact, the sequence of these Superficial Ferns contem- 

 plated here stretches back to a time when the Leptosporangiate type itself 

 was yet in its infancy, as shown by the more complex sporangia. 



The superficial sorus had in the early Mesozoic Gleicheniaceae already 

 settled to a definite type, foreshadowed by the Palaeozoic Oligocarpia 

 (Vol. II, Fig. 493). This was repeated in Laccopteris and Matonidium, and it 

 is represented to-day (though with a distal indusium) in Matonia. These 

 were all Simplices, and the sorus was radiate. But we see in certain living 

 Gleichenias how the centre of the radiate sorus has been filled by additional 

 sporangia, even to the point of a mechanical dead-lock (Vol. II, Figs. 486-489). 

 For further advance in effective fertility some structural change became 

 necessary, such as enlargement of the circumscribed receptacle, lengthening 

 of the sporangial stalk, diminution of size of the sporangia, or some revised 

 method of dehiscence from the primitive median slit (Vol. II, p. 210). Some 

 combination of these would be still more effective than any one change 

 alone. Matonia, with its simple radiate sorus, adopted an ill-defined lateral 

 slit. In Dipteris, with its crowded sorus, the rupture is more definitely lateral, 

 though the annulus is still a complete ring (Vol. II, Figs. 571, 576). The like 

 is seen in Lophosoria (Vol. II, Fig. 551). But while Gleichenia, Matonia, and 

 Lophosoria have all retained a circumscribed central receptacle, in some 

 species of Dipteris this has been widened to accommodate more sporangia 

 {D. Lobbiand), or the sori themselves have been multiplied {D. conjugata, 

 Vol. II, Fig. 573). Yet another feature of advance is seen in D. conjiigata, for 

 here the simultaneity of origin of the sporangia characteristic of the Simplices 

 is given up, and a slight succession of age introduced (Vol. II, Fig. 575). A 

 still greater advance in soral elaboration, involving both the innovations last 



