Natural History of Molluscous Ariimals. 351 



coast is seen, where a heap of empty shells does not form a pro- 

 minent feature near the door : but, although they are gathered 

 in vast numbers, their rapid propagation affords a constant 

 supply, and they do not diminish, comparatively, as some 

 other tribes of shells. 

 Guernsey^ Nov. 1830. 



Art. XII. An Introduction to the Natural History of Molluscous 

 Animals. In a Series of Letters. By G. J. 



Letter 7. On Burrotving and Stationary Mollusca. 



Sir, 



I SHALL continue in this the subject of my last letter, and 

 proceed to make you acquainted with such habits of the fixed 

 and burrowing Mollusca as have relation to locomotion ; and 

 there is more to interest us here than may be at first obvious. 



The permanently stationary tribes are to be found only 

 among the bivalved and tunicated classes, and of the latter 

 the greater number are so circumstanced. Some of these 

 (Distoma, Aplidium, Polyclinium, Botryllus) envelope the 

 stems and leaves of sea-weed in a jelly-like mass, studded 

 over with stellated figures ; others (Cynthi<^) adhere to them 

 or to rocks by a base more or less extended, and, being 

 covered with a rough coriaceous skin, very much resemble 

 large warts or tumours; while a few (Boltenia, Clavelina) 

 rise on a slender stalk which serves them for a cable, and is 

 fixed to its stay by fibrous radicles, not unlike those of some 

 sea- weeds. The adhesion of all of them is permanent and very 

 strong ; so that when you attempt their removal, it must 

 be made cautiously, and with a knife, otherwise their body, 

 tough as it is, will certainly be torn. These Mollusca, how- 

 ever, are born free, and in their veriest infancy even swim at 

 large ; but, unapt to roam, and careless of liberty, they soon 

 voluntarily root themselves beside their parents, never again 

 to remove from the natal rock. 



The bivalves which are stationary are much less numerous 

 in species than the locomotive tribes; but among the former 

 there are some, as the oysters and muscles, which in the number 

 of their individuals probably far exceed all others. With 

 the exception of the two genera A'rca and Chama, they all 

 belong to an extensive suborder, which has only one, or at 

 least one principal, transverse muscle to close the shell with ; 

 and the shell of them all is more or less of the foliated struc- 



