Art. VL — Notes on Didymograptus caduceus, Salter^ ivith 

 Remarks on its Synonymy. 



By T. S. Hall, M.A. 



Demonstrator and Assistant Lecturer in Biology in the University of 



Melbourne. 



[Eead November 14th, 1895.] 



This species is very well known to Australian geologists, its 

 occurrence in Victoria having been announced by Professor Sir 

 Frederick McCoy in 1861,* and in 1875 he figured it in his 

 Prodromus of the Palaeontology of Victoria,! giving at the same 

 time a very full and careful description, and enumerating several 

 of the variations under which it presents itself. 



While it is fairly constant in form, it shows a great range of 

 variability in several points, such as the width of the stipe, the 

 number of hydrothecfe in a given length, and the size of the 

 sicula. In one point I have, however, not been able to confirm 

 the exactness of the figures in the Prodromus, though, as will be 

 presently seen, I do not deny the possibility of the occurrence 

 of specimens exhibiting this feature. 



The examination of a large series has shown that the first two 

 hydrothecje formed are in contact by their inferior margins for 

 from about a quarter to three-quai-ters of their length, leaving 

 between the outer extremities of these margins a more or less 

 deep, acute, V-shaped space, the apex of which is rounded, 

 probably by an extension of the periderm between the two 

 hydrothecje. In one example from Castlemaine the margins only 

 of tlie hydrothecfe are pi-eserved in this region, being shown as a 

 fine black line. In this specimen the concrescence of the mai'gins 

 of the first two hydrothecje is clearly seen for a portion of their 

 length ; they then diverge, leaving between them a space which 

 is acutely pointed below, no extension of the periderm being 

 seen. From its position, immediately over the broad extremity 



« Exhibition Essays, ISGl, p. IGl, reprinted in A.M.N.H., vol. ix., lSt'2. 

 ■f Decade II., plate xx., figs. .3, 4, f>. 



