1869] Dr. Wyville Thomson on Holtenia. 33 



apparently full-grown specimens are from 9" to 1' 1" in length, and from 7" 

 to 9" wide. The outer wall consists of an open, somewhat irregular, but 



very elegant network, whose skeleton is made up of large separate siliceous 

 spicules. These spicules are formed on the hexradiate stellate type ; but 

 usually only five rays are developed, the sixth ray being represented by a 

 tubercle. To form the framework of the external wall, the four secondary 

 branches of the spicule spread on one plane, the surface of the sponge, 

 while the fifth or azygous branch dips down into the sponge-substance. 

 This arrangement of the spicules gives the outer surface of the sponge a 

 distinctly stellate appearance, the centres of the stars being the point 

 of radiation of the secondary branches of the spicules. These quinque- 

 radiate spicules measure about 1" 5"' from point to point of the cross- 

 like secondary branches, and the length of the azygous arm is from 

 7\V" to 1". 



VOL. XVIII. D 



