STRUCTURE AND GROWTH. 277 



shown in the ilkistrations. They are fixed, by means of car- 

 tilages, on the back of the hinge. 



The ckss of shells named Ttihicolce, by De Blainville, are 

 bivalves attached to, or lying within, a shelly tube or pipe; 

 and in the case of the AspergiUum, or watering-pot, the 

 nuclei of the two valves are soldered into the tube so as to 

 form a part of it. This last however is not a British ex- 

 ample. The Anomia has a bony button, which, fixed to the 

 external object of attachment, passes through an opening in 

 the lowest of the true valves ; and many bivalves have little 

 bony appendages placed within, upon their hinges. 



STRUCTURE AND GROWTH. 



Let us take a Univalve shell, or one valve of a Bivalve, 

 and examine the manner in which the growth proceeds 

 from the nucleus. The nucleus, or pair of nuclei, as the 

 case may be, is the first formed part, and is generally pre- 

 sent with the young animal in the egg, or within the body 

 of the parent. It is of a more horny and transparent sub- 

 stance than the after-growth, and generally so different in 

 shape as to give very little idea of what it is afterwards to 

 become. It may be considered as the top, or cqjex, of a 

 sometimes straight or curved, but generally sjnral, cone. 



