ORDER III 



SILICISPONGIAE— LITHISTIDA 



55 



Polyjerea From. ; Astrodadia, Thecosiphonia, Colymmatina Zitt. ; Twronia Mich. • 

 Plinthosella Zitt. (Fig. 54). Cretaceous. Discodermia Boc. ; Bhacodiscula Zitt., 

 etc. Cretaceous and Tertiary. 



Ehagadinia Zittel (Fig. 55). Auricular, plate- or bowl- shaped, short- 

 stemmed. Both surfaces traversed by irregular branching furrows, in which 

 the canalicular ostia are situated. Skeletal elements four-rayed, sometimes 

 uniformly or only distally covered with tuberculous knobs, and with digitate 

 extremities. Dermal spicules in the form of six-lobed disks, provided with a 

 short shaft, and minute, multifid tetraclons. Upper Cretaceous. 



Suborder 2. EUTAXICLADINA EauflF. 



Skeleton composed of four-rayed spicules with three equoMy developed simple or 

 bifurcate rays which terminate distally in root-like fibres ; and one abbreviate, inflated 

 fourth ray {ennomoclon). Axial canals probably in all of the rays. Skeletal elements 

 invariably arranged in either parallel or alternating roivs, and united by zygosis into 

 a network with triangular or irregular meshes ; spicular nodes greatly inflated. 



Nearly all the genera are Silurian ; a few [Mastosia, Lecanella) occur in the 

 Upper Jurassic. 



Fic. 56. 



Astylospongia praemorsa (Goldf.). In erratic block from Mecklenburg, a, Sponge, partially cut into, natural 

 size; 6, Skeleton, 1-/1 ; c, Portion of same highly magnified. 



Astylospongia Eoem. (Figs. 56, 57a). Spherical, with shallow depression on 

 the summit ; base evenly rounded, unattached ; probably fastened by means 

 of anchoring fibres. Large-sized canals directed parallel to perii:)hery in the 

 outer portion of the body, vertical in central 

 portions ; besides these there are numerous fine 

 radial canals which terminate in pores all 

 over the periphery. Skeletal elements with 

 four smooth elongated rays, one or all of 

 which branch dichotomously just above the 

 junction with the shorter arm. Spicular 

 nodes thickened into large knots. Ordovician 



of the Eussian Baltic Sea Provinces, and ment of i/i/irfj«, so/^ (after Rauff). 

 Silurian of Sweden and North America (not- 

 ably in Tennessee), usually chalcedonised. Also in erratics in the Diluvium 

 of Northern Germany. 



'MJ 



Fio. 57. 



Detached skeletal element of Astylo- 

 spongia, 120/1 ; &> Detached skeletal ele- 



