HALLE, ON THE SPORANGIA OF SOME MESOZOIC FERNS. 23 



following figures were obtaiiid: 70, 02, ll'>, 120, 125. All 

 these figures fall between the typical numbers 64 and 128, 

 and as it was very easy to separate and count the spores, it 

 is fairh' certain that the number of spores in each sporangium 

 corresponds to the typical number of 128. 



As might be expected from a comparison of the size of the 

 sporangia, the spore-output is thus much smaller in Thau- 

 matopteris Schenki than in DictyophyUum exile. This dif- 

 ference is another point in favour of keeping the two genera 

 separate. 



Dictyophyllum, Hausmannia and Thaumatopteris are usu- 

 ally held to belong to a fairly natural group of Mesozoic ferns. 

 Seward refers this group to the Dipteridinae to which they 

 present a general resemblance of habit and in some soral cha- 

 racters. In the Dipteridinae, as in the Polypodiaceae gener- 

 ally, in wdiicli great family Dipife-ris was formerly included, 

 the number of spores per sporangium is very constantly 64. 

 In the fossil group there is a much greater variation. Haus- 

 mannia Forchlmmmeri has the same typical number of 64 

 spores, as in Dipteris, or perhaps sometimes 128. In Dictyo- 

 phyllum exile, at the other end of the series, the spore-output 

 seems to correspond in most cases to a typical number of 512. 

 Thaumatopteris Schenki, with a probable typical number of 

 128, occupies an intermediate position, agreeing, howev^er, in 

 regard to the spore-output, more closely wdth Hatismannia. 

 Of the three genera Hausmannia in other respects also, as for 

 instance in regard to the sori and the general habit, is the one 

 most closely similar to Dipteris. All three genera are roughly 

 contemporaneous, but Dictyophyllum is perhaps the oldest, 

 and Hausmannia certainly has its maximum development 

 at a later epoch than the others, being particularly abundant 

 in the lower Cretaceous, It is not suggested that the three 

 genera necessarily are on the same line of descent, but it is 

 of some interest to note the great variation of the spore-out- 

 put, as the recent Dipteridinae to which these fossil generally 

 have been referred are characterized by a marked uniformity 

 in this respect. 



