PORIFERA. 



17 



V. Hydnoceras Conrad. 



Obconical, more or less rapidly expanding from an acute base. 

 Surface at first smooth except for the reticulated spicular threads, 

 but soon developing eight prism faces and prominent nodes in hori- 

 zontal and vertical rows. These nodes usually lie at the intersec- 

 tion of adjoining prism faces and are connected with one another 

 in vertical rows by short prominent lamellae. Occurs only as in- 

 ternal molds. 



Characteristic of the Chemung. Occurs also in the Portage and 

 Waverly. 



6. H. tuberosum Conrad. (Fig. 25.) Upper Devonic. 



Large. Transverse section usually subelliptical from compres- 

 sion. Horizontal rows of nodes not exceeding four or five in ma- 

 turity and separated by broad 

 concavities. Vertical rows 

 eight and separated by con- 

 cavities. Surface regularly 

 reticulated by vertical and 

 horizontal series of spicular 

 bands. 



Lower and Middle Che- 

 mung sandstones in New 

 York. 



^w\ ^m 



Fig. 26. 



Brachiospongia digitata, reduced 

 (Beecher's mem.). 



VI. Brachiospongia Marsh. 

 Skeleton enclosing a hollow 



central cavity which sends 

 out large hollow arms, closed distally. Wall thin and bearing a 

 net-work of spicules. Ordovicic. 



7. B. digitata (Owen). (Fig. 26.) Ordovicic. 



Arms eight to eleven, radiating from circumference of body ; 

 at first they pass off horizontally, then turning at right angles 

 they rise nearly vertically. Size from six inches to one foot in 

 diameter. 



Trenton of Kentucky. 



VII. Astr,eospongia Roemer. 



Thick-walled, bowl-shaped. Upper surface concave, lower con- 

 vex, without traces of attachment. Skeleton composed of rela- 



