1884.] NATURAL SCIENCES OP PHILADELPHIA. 199 



which the separation of the valves in this genus is effected, as 

 shown in 



Fig. 4, much the larger portion of the annulus with all the hooks (which 

 are formed by expansions of its dermal surface) remaining upon one 

 side, and a smaller part, composed of coarser air cells upon the other. 

 It will be noticed that in Pectinatella the annulus is formed of two 

 distinct series of cylindrical cells, short upon one side of the separating 

 line, several times this length upon the other. The corresponding 

 cells of Cristatella are much more complicated, being formed about 

 numerous transverse lines upon the internal surface. The figures have 

 been carefully drawn by the aid of the camera lucida. 



Fig. 5. Outline views of the skeleton and statoblast spicules of the sponge 

 Meyenia Leidyi : A, the normal skeleton spicule ; B and C, side and 

 end views of the normal birotulate ; a, b, c, the corresponding features 

 as modified by their environment upon the iron pipes as described. 



