Maryland Geological Survey 



197 



curved, concave on the inner side, denticulate on the outer side, and have 

 expanded denticulate extremities. The fourth ray is small and directed 

 towards the exterior. It is often abortive. The whole structure consists 

 of superimposed series of these spicules. The manner in which they unite 

 to form the canals is illustrated diagrammatically in fig. 3. Three 

 spicules, whose centers form the angles of the cell, lie on one level while 

 three alternating spicules^, whose centers are dotted in the diagram, lie at a 

 slightly higher level in the structure, their rays descending obliquely upon 



Fig. 5. — Diagram showing the spicules of Hindia fibrosa uniting to form 

 a hexagonal canal (after Rauff). 



the rays of the first set, the third layer repeating the first, etc. In this 

 manner walls are built up around the canals which are perforated by open- 

 ings, placing the interior of the adjacent canals in communication. The 

 form of an individual spicule and the manner in which it articulates with 

 its neighbors is shown in fig. 4 M-hile the manner in which, they unite to 

 form the walls of the canal is shown in fig. 5.^ Excellently preserved speci- 

 mens showing the inner structure are found at Keyser, West Virginia. 



' Both flgs. 4 and 5 are after Rauff, Sitzber. d. Niederrh. Gesell. in Bonn, 

 Sitz. von Mai, 1886, pp. 165, 167, who drew them in an inverted position in 

 original paper. Fig. 3 is modified from Rauff, fig. 3, p. 169. 



