BIVALVE SHELLS. 287 



twisted. The very young JDentalia and some limpets are 

 spirally twisted. The Yermetus commences with a spiral 

 shell resembling Turritella, which after few whorls grows 

 out into an irregular Serpula-like tube. 



SYMMETRICAL CONVOLUTE UNIVALVES. 



No shell is perfectly symmetrical, but some have their 

 successive volutions coiled up so nearly on one plane as to 

 appear so. In this manner the NautihiSj and the Sjyirula, 

 and the ancient Ammonites, have every appearance of being 

 symmetrical. 



All of these are chamherecl shells; but among the Testacea 

 of Great Britain there are few examples of this peculiarity. 

 The Ccecum, when the mature shell is formed, builds a wall 

 in the cavity behind its body, leaving the hollow of the 

 spiral nucleus empty, so that it falls off. On cutting 

 through a Turritella, septa may be seen across the cavity of 

 the upper whorls, dividing it into several chambers. The 

 tube of one species of Teredo is also chambered. 



BIVALVE SHELLS, 



Consisting of two principal pieces united to and turning 

 upon each other by means of a hinge. In describing, the 



