M. K. A. Zittel 0)1 Foasil LiUiistiJie. 237 



tinct group of Calcispongia3 which has still to be accurately 

 characterized, that can be compared with them. 



In conclusion, the geological distribution of the Lithistidse 

 and the extraordinary constancy with which they have inhe- 

 rited their skeletal characters from the earliest periods of the 

 earth's history, testifies to the high antiquity of the group, 

 and against their origin from, or even near relationship to, the 

 Pachytragidai, which I would regard rather as an aberrant 

 ateral branch of the Lithistidre, if, indeed, any genetic con- 

 nexion is to be assumed between them. 



All these circumstances induce me to regard the Lithistidie 

 as a distinct order equivalent to the Hexactinellidte, and 

 taking its place in the system between the Pachytragida^., 

 Geodiuidce, and Ancorinidee on the one hand, and the Hexac- 

 tinellidai on the other. 



A detailed classification of Lithistidfe has not hitherto been 

 attempted, as the few living genera did not show the necessity for 

 any further subdivision, and with regard to the fossil forms as 

 good as nothing has been known. Carter, in his classification 

 wdiich has already been repeatedly mentioned, certainly gives 

 valuable hints towards a grouping of the living Lithistida, 

 and also indicates that the genus Corallistes of Schmidt is 

 composed of heterogeneous elements; but a systematic arrange- 

 ment, or even a clearer characterization of the individual genera, 

 was evidently not in the intention of the distinguished English 

 spongologist. 



If we bring together the living and the far more numerous 

 fossil forms, we soon see the undeniable necessity of a syste- 

 matic arrangement of the very considerable material. As in 

 the case of the Hexactinellida?, so here, I take into considera- 

 tion, for the characterization of the larger grou])S, in the first 

 })lace the characters of the true skeletal corpuscles, in the se- 

 cond the surface-spicules, and in the third the external form. 

 In accordance with these principles the Lithistida; may be 

 divided into four families (Rhizomorina, Megamorina, Anomo-= 

 cladina, and Tetracladina), and these again in part into 

 several sections. 



Revision and Key to the Determination of the Fossil and 

 Living Genera of Lithistidie. 



Class SPONGIJ^. 



Order LlTHiSTiD^, O. Schmidt, 1870. 



Massive, stony, thick-walled, generally attached siliceous 

 sponges of very various external form. Monozoic or polyzcno. 



