THE SHELLS OF THE GULF OF CALIFORNIA. 



217 



on another coat, and the process goes on alternately till one of the 

 party dies. The warty excrescences thus produced, often with the 

 black foot at the top, if the oyster has died first, present a strange 

 appearance inside the Spondylus valve, till the true cause of them is 

 discovered. The same process from outside the pearl oysters or ear- 

 shells produces the irregular pearls which resemble a human foot, 

 (said to bo a sovereign remedy against gout,) a horse's head, &c. 



Time would fail to describe 

 the heart-shaped burrows of 

 Petricolarobusta, and the cylin- 

 drical cavities of several species 

 of Lithophagus, all of which 

 have their "noses" or breath- 

 ing end of the shells close to the 

 outside. The Lithophagus or 

 Boring-mussel tribe are remark- 

 able for arranging the abraded 

 matter outside the horny skin 

 enveloping their fragile shells; 

 each species according to a fixed 

 pattern. One of these, before 

 mentioned as being found abund- Paraphoias, in situ 



antly On the West COast Of Spain «, a, Spondylusvalve broken across; b, the shell in its 



■i" * r • -i i r burrow; c, the cup laminae ; rf, d, accessory valves; e, por- 



and Ainca, OUt not (SO iai' as tion of the shell which gapes in the adolescent state, filled 



l. lm ,,,v,\ in +U.-, flnrihliOQTi farmo U P '" ""' ad,,lt ; A irregular foot cavity; g, g, gray lining 



KUOWIi; 111 mo OdllUUeail IdUIld, filling up previous excavations; h, siphon pipes; \\ inha- 



makes a long pair of twisted lentand expeiiam cm-rents. 



prongs, which look (when we examine the shell alone) as though 

 they were formed for the express purpose of boring; unfortunately, 

 however, like analogous "boring plates" in the ship- worm, they 

 are situated at the outer, not the inner portion of the burrow. They 

 seem connected only with the breathing apparatus. One new type 

 <Jf Boring-mussels was found having siphon pipes like Gastroclnena, 

 but smooth inside (Leiosolenus.) 



After these borers have accomplished their work of destruction on 

 the hard fabric of the oyster banks, other bivalves come and live in 

 the empty burrows, which they are often thought to have themselves 

 excavated. They are, however, more truly nestle rs than borers; and 

 as several individuals often crowd themselves into a narrow cavity, they 

 become very irregular in form, and have been divided up unnecessa- 

 rily into species. Such are the Saxicavre, Cumingias, Sphasnia, etc., 

 as well as the more regular Kelliadse and Diplodontidse. Occasionally 

 several different kinds are found, one inside the other, each having 

 gone to live inside the skeleton of his predecessor. Thus nature fills 

 up death with life. 



But the boring bivalves are not the only, perhaps not the principal 

 agents of destruction on the oyster valves. The whole colored layer 

 is generally found riddled with a labyrinth of galleries excavated by 

 humble worms, and still more humble perforating sponges. These 

 galleries again, after the death of their makers, form the pasture 

 fields for many tiny species of Gasteropods, the largest of which are 



