M. K. A. Zittel on Fossil HexactinelUda. 405 



species) was proposed by Cabanis and Heine in the 'Museum 

 Heineanum' (Th. iii. p. 35, 1860), without any characters to 

 designate it having been given, to contain about half of those 

 species originally placed in the genus Amazilia ; and for the 

 remaining portion the term Hemithylaca was instituted, — these 

 names to be taken as substitutes for the less-classical ones of 

 Amazilia, Erytkronola, Saucerottia, &c., by which various 

 species of this section of the Trochilid^ were generically 

 known. With a tolerably large series of nearly all the 

 known species of these so-called genera to judge from^ I 

 cannot satisfy myself that sufficient characters exist in any 

 one of them to warrant its separation into a distinct generic 

 rank from Amazilia, the term which was first proposed for 

 them, and by which I prefer to call them. It is to be under- 

 stood, however, that for generic characters, the style of colora- 

 tion exhibited by any particular number of individuals, when 

 unsupported by any differences of structure, is not taken into 

 any consideration, since it would appear to be very apt to lead 

 one into difficulties, as is clearly shown, I think, to be the 

 case, judgmg from the unsuccessful efforts of those authors 

 who have endeavoured to arrange these birds into different 

 genera, whose characters were mainly those of colour. 



I have not, therefore, adopted the Pyrrhoplioena of MM. 

 Cabanis and Heine, even in the restricted sense in which it 

 has been employed by some ornithologists, deeming it a 

 generic term not required to designate even that particular 

 section of Humming-birds to which the new Amazilia lucida 

 belongs. 



LIII. — Studies on Fossil Sponges. — I. HexactinelUda. 

 By Karl Alfred Zittel. 



[Continued from p. 273.] 



Personal Observations. 



If it be possible to use the skeleton-spicules as the basis of 

 a system, such a system ought most certainly to express most 

 clearly the inherited peculiarities, and consequently the natural 

 relationships of the HexactinelUda. If less attention has 

 hitherto been paid by zoologists to precisely these true skele- 

 ton-spicules than to the flesh-spicules, this was evidently due 

 to the uniformity in their skeletal structure possessed even by 

 rather distant genera. In the flesh-spicules the differences 



