LECTURES ON MOLLUSCA. 117 



and fixed, it is found to have left both the early free valves, having 

 fastened them on the right valve, and deposited layer upon layer over 

 them. At the same time the adductor muscles have united so as to form 

 only one scar. Lamarck made his primary division of the bivalves 

 into those with two and those with one adductor muscle. This creature 

 would have had to march from one to the other order, as he approached 

 maturity. The entire withdrawal of the animal from one valve and 

 manufacture of another is a complete anomaly. It is greatly to be 

 desired that some New Granadian would watch the development of 

 the animal. 



Family MYTILID^I. (Mussels .) 



The Mussels are easily recognized by their triangular shells, which 

 are generally pointed at the anterior, and very much produced at the 

 posterior side. The Mytilus edulis is much used for food in some parts 

 of England, and is found widely diffused in the northern hemisphere, 

 being taken on both sides of the Atlantic and the Californian coast. 

 About 400,000 are eaten every year in Edinburgh alone, and enormous 

 multitudes are collected for bait. In Mytilus the mantle is freely open, 

 fringed in the breathing region like Unio ; and the small foot is grooved 

 to spin a stout byssus by which the animals attach themselves to rocks 

 or to each other in enormous numbers. The shell of Myrina resembles 

 Alasmodon, and was found on floating blubber. 



The shell of Modiola is swollen near the hinge ; and the mantle is 

 partially closed into an excurrent tube. The animal spins a very fine 

 byssus, in which it sometimes wraps itself up. Crenella has a swollen 

 transverse shell, always furrowed outside and crenated within. The 

 hind part of the mantle is produced into an excurrent tube, and it is 

 partially closed in front. The animal spins for itself a silky nest, or 

 burrows in the test of Ascidians. The shells of Litliopliagus are finger 

 shaped and very thin. They burrow in rocks, shells, and corals, the 

 hole being only just large enough to receive them and not to turn round 

 in. The outside end is generally encrusted with spongy layers, of 

 different arrangement in different species, often produced into long 

 beaks, but always outside the skin, and capable of being separated 

 from the rest of the shell. These beaks sometimes interlock ; but have 

 no more to do with the burrowing than the pallets of the shipworms. 



Fossil Mussels are found in all ages from the paleozoic times. Those 

 from the old rocks have been grouped under Modiolopsis and Ortho- 

 notus. 



Family DREISSINIDJE. (Closed Mussels.) 



These differ from the true Mussels, as Iridina does from Anodon. In 

 the fresh-water Dreissina, which was accidentally brought on timber 

 from Russia to London, and is now completely naturalized all over 

 England, the mantle is closed all round, and produced into two short 

 breathing pipes, with an opening for the byssus-spinning foot. The 

 shell differs from the true Mussels in having a deck at the beak to sup- 

 port the anterior adductor muscle. The same deck is seen in the 

 marine Septifer, and in the fossil genera, Hoplomytilus and Myalina. 



