426 MOLLUSC A. 



chara, which generally forms thin crusts, composed of poly- 

 gonal cells, may be taken as the type of this group, and 

 appears to be allied to the recent Flustrce. Some of the 

 forms which have been referred to Monticulipom will doubt- 

 less find their true place in this Polyzoan genus. 



Still another group of the Palaeozoic Polyzoa is typified by 

 the Hippotlwa of the Lower Silurian (fig. 268, c), a genus still 

 in existence. In this form the ccencecium is parasitic and 

 decumbent, and consists of rows of pyriform cells, each with 

 a single opening on its front face, lateral branches similarly 

 composed being given off from the sides of certain of the 

 cells. 



Fig. 268. A, Portion of the crenoeciurn of Alecto auloporoides, of the natural size and en- 

 largedLower Silurian ; B, Portion of the crenoecium of Ceramopora Huronensis, of the 

 natural size and enlarged Devonian ; c, Portion of the coenoacium of Hippothoa inflata, of 

 the natural size and enlarged Lower Silurian. (Original.) 



Lastly, we have a group of Palaeozoic Polyzoa, typified by 

 the genus Alecto (fig. 268, A), and belonging to the Cyclosto- 

 matous division of the class, and to the family Tubuliporidce. 

 In this form the ccencecium is parasitic on foreign bodies, 

 and the cells are tubular, with round mouths, and disposed 

 either in single series or in irregular transverse rows. 



In the Ceramopora of the Silurian and Devonian forma- 

 tions (fig. 268, B) we have another Cyclostomatous genus, 

 with relations to both Alecto and Diastopora. The ccencecium 

 in this form has the character of thin, generally circular, 



