122 Fossil Corals, — Lower Silurian RocJcs o/* Canada. 



under consideration, each individual grew separately and unconnected with 

 any other. Fig. 5 shews what we suppose was the appearance presented by 

 one of these Polypi when growing on the bottom of the ocean. Outside, it 

 probably consisted of a soft fleshy covering, which attached itself by a spread- 

 ing base to the bottom. This soft integument also spread over the top and 

 w-as perforated in the centre by a small opening, which was the mouth. — 

 Around this was the circle of tentacles ; fi'om the mouth there hung down 

 into the interior a small sack, which was the stomach ; between this and the 

 exterior there were a number of thin partitions radiating in the manner 

 shewn in Fig. 6. These partitions and the inner portions of the exterior 

 envelope or sack became solidified during the life of the animal, in the same 

 manner that the bones of a quadruped are formed within the exterior soft 

 covering of flesh. All those corals which are to be seen in the cabinets of 

 the curious, were, when alive, covered with a thin gelatinous layer of fleshy 

 substance. After death this decays, and only the solid part, or the coral, 

 properly so called remains, preserving the shape of a branching twig, a dome 

 shaped mass, or a cup, according to the species. The corals of this extinct 

 genus Streptelasma are of the latter form, and partly hollow within, though 

 usually found filled with limestone. Good empty specimens shew the radiat- 

 ing partitions projecting inward and meeting in the certre at the bottom of 

 the cavity. The partitions or lamelte, as they are called, extend up and 

 down, and are more numerous above than below. 



With the above explanations it will perhaps not be difficult for the 

 student of Canadian Geology to understand the following concise description 

 of the genus. It will be recollected that a family of animals, or fossils, con- 

 tains a number of genera, and each genus, one or more species. 



GENUS STREPTELASMA, (Hall.) 

 Generic Characters. — Corallum, simple, turbinate ; radiating lamellsp, 

 meeting in the centre at the bottom of the cup, where they are more or less 

 twisted ; no transverse diaphragms. 



The generic name is from the Greek [StrejJtos,) twisted; and {plasma,) 

 lamella ; corallum means simply " coral ;" turbinate, is top-shaped or conical. 

 This genus is also called (Petraia) by many European Geologists, from the 

 Greek (Petraion,) stony, or living among stones. 



Thei-e are several species of this genus {Streptelasma) in the Trenton 

 and other limestones of Canada. They v;sually have the appearance of short 

 curved petrified horns of some ruminating animal. They are striated upon 

 the outside from the top to the bottom, each of the striae marking the posi- 

 tion of one of the lamellse inside of the cup. The following are the specie 

 that most frequently occur in Canada : — 



Streptelasma cornicltlum, (Hall.) 

 This species is generally from an inch to one inch and three quarters in 

 length, consideralDly curved and marked by several obscure wrinkles or folds, 

 between wliich again are many finer ones that encircle the cup. These ars 

 only visible in perfect specimens. Those which are worn on the outside do 

 not shew them. In the specimens in our collection which we believe to 



