REPORT ON THE HEXACTINELLIDA. 11 



II. Sexradiate skeleton spicules, with rays making any angle 



■with each other. Aphrocallistidse, . . Aphrocallistes, Dadylocalyx, Ip- 



Jiifeon, Siromafojiora (Callo- 

 dictyon, Sollas, n. gen.) con- 

 centrica. 

 III. Skeleton spicules, cemented into ladder-like fibre. Eu- 



plectellidre, ...... Eapledella, Sympagella. 



Zittel's epoch-making work on fossil Sponges' contains very accurate descriptions, 

 not only of the form of the body and nature of the surfaces, but of the system of canals 

 that penetrate the body, and especially of the fine microscopic structure of the siliceous 

 framework. The flesh spicules which lay loosely in the soft tissues, and were thus, for 

 the most part lost, could not of course be so closely studied. 



As the main basis of his classification Zittel emphasised the differences in the modes 

 of union exhibited by the skeletal spicules, a basis of division which had been already 

 employed by Saville Kent and Carter. He distinguished those forms " in which the 

 skeletal spicules usually I'emain isolated and are only united by sarcode," from those 

 " in which the skeletal spicules are fused in a regular manner and form a continuous 

 lattice-work with cubical or polyhedral meshes." The former he named Lyssacina, 

 the latter Dictyonina. 



That intercommunication of the lumina of the axial canals throughout all the 

 spicules fused into the lattice-like framework, which had been observed by Marshall in 

 Sclerothamnus, was not corroborated by Zittel, either in any fossil Hexactinellid or even 

 in Sclerothamnus itself. It seemed to him, further, that the formation of a special 

 group of Monacid*, in Marshall's sense, was unwarranted, at least as regards the 

 division of the Dictyoninse, but he himself formed, within the Lyssacina, from certain 

 fossil genera, a similar group, and ranged alongside of it the Pleionacidse and 

 and PoUacidse. 



I will here cite the fundamental principles of Zittel's Hexactinellid system of 

 1878 :— 



Class Spongia. 

 Order HEXACTINELLIDA, 0. Schmidt. 



Siliceous Sponges with six-rayed spicules, isolated or fused into a lattice-work of a 

 hexradiate pattern. All the siliceous elements exhibit the same fundamental structure, 

 with an axial cross formed by three central canals intersecting at right angles. In 

 addition to the peculiar skeletal needles there are numerous isolated flesh spicules, mostly 

 very delicate in form. 



^Abhandl. d. II. CI. h. barer. Akad. d. Wi^s., xiii., 1878. 



