S18 



Silurian Fossils. 



layers, shells and stones on the bottom of the sea. Concerning 

 their structure, little can be said in this place. Each one of the 

 minute cells to be seen in the fossils is the cavity once occupied 

 by the viscera of a single Bryozoon. Minute as these animals aro, 

 yet each individual of the recent species is found to possess a mouth 

 surrounded by about twelve tentaeula covered with vibratile cilia, 

 or exceedingly fine hair like filaments, which by their constant 

 motion cause currents in the water, and assist in capturing food. 

 The food passes from the mouth into a gizzard, whence after having 

 been comminuted, it is conveyedinto an elongated stomach, and there 

 dio-ested. From the stomach an intestine proceeds to the surface, 

 and opens near the mouth of the animal, serving to discharge the 

 undigested portion of the food. 



Ptilodictya acuta, the most abundant Bryozoon of the Trenton 

 limestone, is represented by Fig. 5, copied from the first vol. of 

 the Palseontology of New York. The branches are about one 

 eiffhth of an inch in width, and from one to four inches in length. 

 They are flat and rather sharp at the edges. There are from six 

 to ten rows of cells. A narrow pace on the edge of each branch 

 is without cells. 



Fig. 6. 



Fi^. 7. 



Fig. 8. 



Fig. 6. RapMstoma staminea. — Hall. View of the top of the shell. 

 Fig. 1. Raphistoma staminea. View of one side. 

 Fig. 8. Lingula quadrata. 



Ptilodictya is from the Greek ptilon, a wing, " more especially a 

 membraneous wing as that of an insect" and dictuon, a net, acuta^ 

 Latin, sharp, in allusion to the sharp edges of this species. It was 

 called Stictopora by Professor Hall, from stictos, spotted, and 

 pora, a pore, but it is now thought to belong to the first named 

 genus. It is common in the Trenton limestone, and also in rocka 

 of the same age in Wales. 



