122 M. K. A. Zittel on Fossil LitMstidce. 



or oblique direction from the centre towards the periphery 

 and open at the surface as fine pores. These canals exist in 

 great number and are pressed close together and never rami- 

 fied ; they give the sponge a fibroid structure in transverse or 

 longitudinal sections. Frequently the canal-system of the 

 preceding group is combined with these radiating radial 

 canals. As typical genera of this kind may be mentioned 

 Coelocorypha, Scytalia, and Pachimon. 



The canal-system becomes rather more complicated in the 

 fifth group, to which Aulocopi'urn, Siphonia, and some allied 

 genera belong. In these, curved canals of considerable size, 

 which are at first parallel to the periphery, but become almost 

 perpendicular towards the middle, open into the funnel-shaped 

 stomachal cavity. Besides these bowed canals, simple, straight, 

 radial canals of the same or smaller size run in an oblique 

 direction from within outwards : their number is in inverse 

 proportion to their diameter ; so that in forms with thick radial 

 canals {Sij^homa, Melonella) comparatively few are present, 

 whilst sometimes {e. g. in certain Aulocopia), by their capil- 

 lary nature and closely approximated position, they almost 

 give rise to the appearance of a fibrous structure. This modi- 

 'fication of the canal-system has already been admirably figured 

 by F. E-oemer * in the genus Aulocojnum, by Quenstedt f in 

 Melonella, and by Sowerby J in Siphonia. 



A very characteristic form of canals in the LithistidtB are the 

 vertical tubes, which have already (p. 117) been described. 

 These frequently appear to replace the central cavity {Jerea^ 

 Jereica, Stichophyma, Carterella). They are either collected 

 into bundles, or are more isolated and traverse the whole 

 length of the sponge-body in the form of round tubes ; in 

 ramified stocks the principal stem and all the lateral branches 

 are penetrated by such tubes. Their walls are usually fur- 

 nished with pores, the apertures of fine radial canals. If the 

 skeleton is of very loose texture and the vertical tubes are 

 closely approximated, the latter may acquire a polygonal 

 section, when they are generally separated from each other by 

 thin walls [HyalotragoSy Fyrgochonid) . Radial canals of the 

 most various kinds may be combined with these tubular 

 canals. 



A last type of canal-system seems to occur, so far as I 

 know, only in a few Lithistidaj. In these the entire, usually 



* Die fossile Fauna der silurischen Diluvialgescbiebe von Sadewitz, 

 Taf . ii. fig. 1% 2^ 3^ and Taf. iii. fig. l^ 2\ 



t Petrefactenlmnde Deutschlands, v., Taf. 126. figs. 61, 62, 63. 



\ Fitton, " Strata below the Chalk," Geol. Trans, ser. 2, vol. iv. pi. xv". 

 figs. 4-7. 



