HALF AN HOUR WITH SHELL-FISH (BIVALVES). 207 



formation. If so, then it must have had a con- 

 tinuous existence since that distant epoch a period 

 of time that is inconceivable, and that cannot be 

 reckoned in years. The latter mollusc (Crania) 

 takes its generic name from the supposed resem- 

 blance of the upper valve to a helmet. It is 

 peculiar to our northern seas, as its specific name 

 implies, and is not uncommonly met with off the 

 shores of Scotland, during dredging operations. 

 The Crania, as a rule, are found parasitically 

 attached to other objects. 



Although the brachiopods are classed among the 

 bivalves, they really possess a nearer affinity to the 

 " Sea-mats," or Polyzoa. A common name for them as 

 a group is that of " Lamp-shells," on account of the 

 resemblance of the closed valves to an antique lamp. 

 The ventral valve is generally the larger, and the 

 beak is perforated. Out of this perforation, in the 

 living animals, there passes a kind of plug, or 

 peduncle, the end of which attaches itself to stones, 

 &c., and thus anchors the animal and its shell in 

 safety. As far back as we can go in geological 

 time, we find the " Lamp-shells " with perforated 

 beaks. The interior of the shell is occupied with 

 two long, coiled-up processes called " arms," which 

 can be uncoiled and used for obtaining food. They 

 also form the means by which the creature can 

 breathe or respire. The interior shows the mouth, 

 which is situated at the base of these arms ; gullet, 

 stomach, and liver. As is frequently the case with 



