204 Mr. E. A. Minchin. On the Origin of [June 13, 



and Cycadese. The view of their affinities, which we suggest, is that 

 they are derivatives of an ancient generalised race of ferns, from 

 which they have already diverged considerably in the cycadean 

 direction. Of the two genera, Heterangium appears to be geologi- 

 cally the more ancient, and certainly stands nearer to the filicinean 

 stock. Lyginodendron, while retaining conspicuous fern-like charac- 

 ters, has advanced much further on cycadean lines. This view by 

 no means involves the improbable assumption that these plants were 

 the actual ancestors of existing Cycadeae. How far their divergence 

 from, the fern stock had proceeded cannot be determined until we 

 are acquainted with their organs of reproduction. 



The existence of a fossil group on the border-land of ferns and 

 Cycads seems now to be well established. Count Solms-Laubach 

 places his Protopitys in this position, which is probably shared by 

 Myeloxylon and Poroxylon. Messrs. Bertrand and Renault have 

 indeed endeavoured to derive the last-named genus from Lyco- 

 podiaceas, and have extended the same view to Lyginodendron and 

 Heterangium. In the latter cases their theory is completely negatived 

 by the organisation of the leaves, and by many structural details. 



The relation of the genera which we have described to those 

 ancient gymnosperms, the Cordaitece, will form one of the most 

 interesting palaaobotanical problems of the future. 



The paper is illustrated by micro-photographs and by camera- 

 lucida drawings. 



IV. " On the Origin of the Triradiate Spicules of Leucosolenia." 

 By E. A. MINCHIN. Communicated by Professor LANKESTER, 

 F Jl S. Received April 30, 1895. 



(Abstract.) 



In Leucosolenia coriacea the youngest spicules are found to be 

 surrounded by six cells, which are similar in all their characters to 

 the cells of the external flat epithelium of the sponge, and undoubtedly 

 derived from this layer. It appears that three cells of the external 

 epithelium wander inwards, and give rise to six by division of each 

 cell into two, the six cells being arranged in such a way, that three 

 are placed more internally, i.e., towards the gastral surface of the 

 body wall, and three more externally, towards the dermal surface. 

 Each of these sets of three cells has a form which might be compared 

 to a trefoil, and the whole mass may be described as two such trefoils 

 superposed, the cells of one trefoil exactly corresponding to those of 

 the other. 



The spicule is formed by the three inner cells, a ray being formed 



