34 



DiOTYOSPONO IT) JR. 



regularly olK-onical. In IIydnocebas this primitive, smooth condition was of 

 brief duration and was directly followed by the development of prism-faces, 

 at first four and directly thereafter eight, the nonnal number. These pi-ism- 

 faces are well developed before the appearance of the nodes on the prism- 



FiouKE 8. Primary and modified forms of the Hexactio. 



a. Oxyhexactin, x 100 ; 6. Ecliinhexaotin, x 300 ; c. StronBylhexactln with prickly extremitiea, x 450 ; 

 d. Tylhexaster, I 300 ; e. Discuhexaster, x 450 ; /. Plumicome, x 300 ; g. Amphidisk, with remnants of the 

 abbreviated arms, x 100 ; h. Pinule, x 300 ; t. Orthotetraen, x 100; j. Clamle, x 450 ; k. Qiaen, x 150. 



(Rauff.) 



angles. The nodes themselves do not appear abruptly but with gradual 

 increase in size, and after the passage of the adult period their size again 

 decreases, and eventually they disappear; the prismatic faces also become 

 obsolete, the apertural portions of old individuals being smooth and funnel- 

 shaped. 



The remarkable genus Botbyodictya retains the smooth, primitive, 

 subcylindrical form of the cup for fully one-half its length and then abruptly 

 swells into a vase with obscure prism-faces and an extravagant development 

 of compound pouch-shaj^ed nodes. In the great majority of all of the Devonian 

 species of these sponges the prism-faces are more or less clearly discernible, 



