CYCLOSTOMATA. 



621 



more or less circular, and are often provided with a projecting lip or 

 " peristome." 



The minute structure of the polyzoary in the Fenestettida appears 

 to be verv uniform. The cells themselves are formed of a layer of 



j?\\ \ ' ' /f/ 





":WK.'- 





Fig. 466. — Fenestella. Lyelli. a. Natural size ; b, Portion enlarged ; c, Cells and spines in 

 profile. From the Carboniferous rocks of Canada. (After Dawscn.) 



perfectly homogeneous compact calcite, which is strengthened by a 

 basal striated membrane ; while the reverse or non-poriferous side 

 of the polyzoary is composed of a thick stratum of calcareous tissue 



Kfnm 





MM 



Z*~-*l 



i^g@g*ff 



'CfQQO 





Fig. 467.— Minute structure of Fenestella tuberculocarinata, from the Carboniferous rocks of 

 Scotland, a, Section taken parallel to the non-poriferous surface, showing thickened tubes (for 

 the support of " avicularia " ?), enlarged. B, Section parallel to the celluliferous surface of the 

 polvzoarv. showing the biserial cells (c) : d, Dissepiments ; f, Fenestrules. (Original.) 



traversed by exceedingly minute tubuli, the direction of which is 

 perpendicular to the surface, and which communicate to thin sec- 

 tions of this region of the skeleton an exceedingly characteristic 



