'4 



forma platydisca. 



II. platydisca Decne. 

 I pper joints large, thin ; reniform, discoid or transversely oblong (fig. 2). 



forma Albertisii. 



II. Tuna forma Albertisii Piccone. 

 Joints elongate-cuneate, lower ones often deeply calcified and thick ; upper margin 

 rounded (tig. 3). 



The list of synonomy under this species is a long one. It will be seen that some of 

 the old authors confused H. Tuna and H. Opuntia, but since the two species are themselves so 

 distinct externally it is not difficult to determine to which species the descriptions and plates refer. 



The names of H. platydisca Decne, H. discoidea Decne, H. Lcssouii Bory and H. 

 papyracea Zanard. also find their places here for the first time under H. Tuna, and this may 

 be considered to require explanation. 



Thanks to the kindness of M. Hariot, I have been allowed to examine the type specimens 

 of the first three species, which are preserved in the Paris Museum ; their internal characters are 

 identical with those of H. Tuna and their external characters come well within the rano-e of 

 variation of this species. 



H. platydisca was queried by Dr. Harvey as a synonym for H. Tuna in his Ner. Bor. 

 Amer. vol. III. p. 25. as the result of an examination of specimens brought home by him 

 from Key West. These plants I have been permitted to see, through the kindness of Dr. Perceval 

 Wright, and they form a good series of links between H. platydisca and H. Tuna. Dr. Harvey 

 says of them : "The original H. Tuna is a native of the Mediterranean ; and possibly the plant 

 now described may be referable to H. platydisca Decne, but some of my Key West specimens 

 so closely resemble what I have received from the Mediterranean, that, habitat apart, I cannot 

 find a character to distinguish them. Others are certainly of larger size, with more discoid 

 articulations. If, however, every slight variation in form and size is to be made the foundation 

 of a new species and dignified with a special description and name, the number of species to 

 be established would only be limited by the number of specimens examined ; for scarcely two 

 are to be found identical at all points". Plants collected at Cape de Yerde and Barra Grande by 

 H. M. S. u Challenger" and preserved in the British Museum, confirm this view, shewing 

 gradations in form between H. Tuna and H. platydisca. 



This view of Dr. Harvey did not however find favour with later writers and H. platy- 

 disca was retained as a species. Professor Schmitz, (1. c.) while admitting the general similarity 

 between II. Tuna and II. platydisca, believes them to be specifically distinct on account of 

 the difference in their fruit. The fruits of H. Tuna are described as tufts of sporangiophores, 

 arising from the upper edge of a joint and reaching a length of about 3 — 4 mm. : those of 

 H. platydisca as being distributed regularly along the edge of a joint and reaching a length 

 of about 2 mm. In //. 'Tuna the sporangia are borne laterally on the sporangiophore : in 

 H. platydisca their position is apical. This last character seems the only one of any importance, 



