XXXIV] FOSSILS 321 



the spore-numbers, which go more or less parallel with the size of the 

 sporangia, that the most interesting features of these Mesozoic fossils lie. 

 Halle {I.e. p. 23), after remarking on the constancy of the typical number of 

 64 for Dipteris, notes that Haiismannia Forcliammeri Barth., of Jurassic 

 age, has usually the typical number of 64, but sometimes 128 spores. 

 H. H. Thomas finds in Dictyophyllimi riigosum, a fossil referred to this 

 affinity, in which the sporangia are smaller than in D. exile, the typical 

 number of 128: the fossil comes from the Gristhorpe Beds of the Yorkshire 

 Oolite. In Thajimatopteris Schenki Nath., of Rhaetic age, Halle states that 

 the number is probably 128, while in Dictyophylluni exile Nath., also of 

 Rhaetic age, the typical number is in most cases 512. We need not, as Halle 

 rightly remarks, assume that these fossils are necessarily all on the same 

 line of descent. But it is a matter for special remark that there is a larger 



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I' 



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r , J 



Fig. 579. Hatumannia sp. Upper Jurassic, near Helmsdale, 

 Scotland. From a specimen in the British Museum. (Natural 

 size.) (From Seward.) 



spore-output in them than in the living Dipterids. The largest of all is that 

 oi DictyopJiyllum exile which dates from the Rhaetic period, while the smallest 

 among the fossils in question is that of Hansinannia from the Jurassic. 

 Moreover the larger number goes along with a greater size of the individual 

 sporangium, and a smaller number of the sporangia in each sorus. Referring 

 to the sori of H. exile, Nathorst has been able to state that the number of 

 these in each is probably 4-7 (Halle, I.e. p. 16). This number of sporangia 

 in the sorus is not unlike that found in Matonia, in Laccopteris, Gleichenia, 

 and Oligocarpia. Such facts when taken with the general morphology and 

 anatomy of all the parts, living and fossil, establish a close nexus between 

 them. They make some degree of actual relationship appear much more 

 probable than has been generally realised: and they suggest that the Dipterids 

 now living form a line of later derivation from a stock essentially Gleicheni- 

 aceous in type. 



