244 



OUR COMMON BRITISH FOSSILS. 



bottom where they are protected. These siphons can 

 be lengthened or shortened at the will of the animal, 

 by means of special muscles. These muscles leave 

 their marks on the interior of the shell, so that a 

 naturalist can readily tell a Siphonate mollusc from 

 an Asiphonate kind. In the latter, the scar left by 

 the mantle's attachment is unbroken ; in the former, 

 it is indented into a sort of bay or sinus. Scars are 

 also left by the adductor muscles, which close the two 

 shells and defend the animal from enemies. 



Singularly enough, there is a geological peculiarity 

 about the two kinds of bivalves just mentioned. 





Fig. 227. — Cardiola interrupta 

 (Silurian). 



Fig. 228. — Pterinea sitbfakata 

 (Silurian). 



The siphon-bearing {Siphonate) moUusca get more 

 numerous in the Secondary rocks than they were in 

 the Primary, whilst they are far more numerous in 

 the Tertiary strata than in the Secondary. The 

 Primary and early Secondary rocks are marked by a 

 preponderance of siphonless {Asiphonate) bivalves. 



Fossil bivalves first make their appearance in the 

 Lower Tremadoc rocks, where about twelve species 

 have been found. The univalves first appear in the 

 Arenig rocks. In the neighbourhood of St. David's 



