Silurian Fossils, 



SIT 



in length, and scarcely an incli in diameter at tlie aperture. The 

 form of the species is straight, cylindrical, slender and gradually 

 tapering. The shell is characterized by slightly arched or undu- 

 lating rounded annulations, distant from each other about two 

 fifths of the diameter of the tube. The surface is marked from 

 one end to the other by sharp longitudinal elevated lines, a finer 

 line between every two of the coarser ones; we have never seen 

 the septa of this species, and cannot say how near they are together. 

 The siphuncle is near the centre. 



This species is found at the localities given for Oncoceras con- 

 stricium. The specific name is from the Latin his, twice or double 

 and lineatus, the participle of the verb lineOj to draw lines. 



Fig. 5. — Ptilodictya acuta. 

 Stictopora acuta. — Hall. 



The genus Ptilodictya consists of thin leaf-like fossils, branched 

 and with both surfaces set with numerous small ovate cells. 

 These and other fossils of several allied genera have until recentlv 

 been considered to be corals. They are now, however, thouo-ht to 

 be the remains of animals which belonged to the department of 

 the MoUusca. There are many species in the existing seas, and they 

 are characterized principally by their mode of growth. The 

 ordinary Mollusca consist of individuals free and separated from 

 each other, each one leading an independent existence ; but in the 

 Bryozoa, (Greek, hruon, sea moss, and zoon, an animal,) as they are 

 called from their moss like growth, great numbers grow too-ether 

 forming twigs, leaves or plant-like objects, or encrusting in thin 



