626 



MOLLUSCOIDEA. 



on the outside of the funnel-shaped polyzoary. The remarkable De- 

 vonian genus Unitrypa, again, resembles the two preceding types in 

 general structure ; but the projecting keels which separate the two rows 

 of cells on each branch are widened at their summits, and are connected 

 with one another by thin lateral processes placed at variable distances. 

 In the Carboniferous genus Ptilopora (fig. 470) are included forms which 

 agree in essential structure with Fenestella, but differ in the feather-like 

 form of the frond, the polyzoary having a central stem which gives off 

 lateral branches in a pinnate manner, these latter being connected by 

 dissepiments separated by oval fenestrules. The genus Archimedes 

 (Archimedipord) is another remarkable Carboniferous type, in which the 

 general structure is like that of Fenestella, but the reticulate polyzoary is 

 wound in an obliquely spiral manner round a central screw-like axis (fig. 

 471, c). The genus Helicopora has a similarly spiral zoarium, but there 



Fk 



470. — Ptilopora phuna ; the right-hand figure of the natural size, the left-hand 

 figure enlarged. Carboniferous. 



is no central axis developed. Lyropora, from the Carboniferous rocks of 

 North America, resembles Fenestella in general features ; but the net-like 

 polyzoary is bounded by solid lateral supports, which spring, like the two 

 branches of the letter U, from a small base of attachment. Lastly, the 

 genus Fenestrated, likewise from the Carboniferous rocks of North 

 America, differs from Fenestella principally in the fact that there are two 

 rows of cells on each side of the median keel in each branch. 



The Carboniferous genus Actinostoma resembles FeJiestella in having 

 a fenestrated polyzoary which is poriferous on one side only ; but the 

 branches are not keeled, the cell-mouths are furnished with radiating 

 teeth or " rays," and the zocecia have a second supplementary pore close 

 to the proper aperture. The widely distributed genus Polypora (fig. 469, 

 A and B, and fig. 471, a) agrees with Fenestella in general structure, but 

 the branches are not keeled, and there are from three to six rows of cells 

 to each branch. As in Fenestella, however, the branches are connected 

 by solid, non-poriferous " dissepiments." On the other hand, in the genus 

 Phyllopora (the Retepora of many authors) the branches which compose 

 the funnel-shaped polyzoary are not joined by dissepiments, but are 

 sinuous, and anastomose with one another at short and regular intervals, 

 so as to give rise to a symmetrically disposed system of " fenestrules " 

 (fig. 469, E and f). The zocecia are placed on the inner side of the frond, 



