116 M. K. A. Zittel on Fossil Lithistidce. 



a quadriradiate division of the central canal, held together by 

 amorphous sarcode and an interlocking of their filigreed arms, 

 forming a reticulated glassy structure, whose interspaces are 

 more or less irregular and curvilinear. Composed of two 

 kinds of ' skeleton-spicules,' viz. those which form a layer on 

 the surface and are accompanied by minute or * flesh-spicules ' 

 characterizing the species, and those forming the body, which 

 are more or less alike in all the species and accompanied by 

 fewer flesh-spicules. The skeleton-spicules of the surface, 

 which, for the most part, are provided with a smooth, pointed, 

 vertical shaft, directed inwards, and a horizontal head of dif- 

 ferent shape according to the species, will be termed ' sur- 

 face- ;' and the spicules of the body, which interlock with 

 their neighbours through a filigreed development of all the 

 arms, will be termed ' body-spicules.' " 



Several deep-sea Lithistidaj, dredged up in the Atlantic 

 Ocean by Prof. Wyville Thomson on board the ' Porcupine,' 

 have since been submitted by Carter to an accurate analysis*. 



A. Pomel, in his great workf on the fossil sponges of Oran 

 (pis. A, B, & E), also gives figures of several living Lithis- 

 tidce. Unfortunately, however, an accurate description of the 

 minuter structural characters is wanting to the genera Cisseliaj 

 u:Egophymia, and Pumicia of Pomel ; so that it can hardly be 

 decided with certainty whether these agree with already 

 known forms, or whether they are to be regarded as new 

 genera or species. 



External Form. 



The external appearance of the Lithistidse is exceedingly 

 various, and even within the same genus is by no means con- 

 stant. From the solid stony nature of the skeleton, we might 

 have expected a greater constancy of form than in other 

 sponges ; but, notwithstanding this circumstance, we may 

 apply to the Lithistidas also the principle that the general 

 form only plays a secondary part in the classification of the 

 sponges, and can never be available for the characterization of 

 orders or families. 



The Lithistidse most frequently imitate the forms of basins, 

 cups, leaves, tops, and cylinders, but globular, pyriform, 

 nodular, and amorphous bodies not unfrequently occur, whilst 

 branched and bushy stocks are met with only in a few genera. 

 They are generally adherent. In many the lower part of the 

 sponge-body is developed into a longer or shorter stalk, which 



* Ann. & Mag. Nat. Hist. ser. 4, vol. xviii. (1876) pp. 460-468. 

 t Paleontologie de I'Oran, 1873. 



