Ch. III.] 



INDICATED BY FOSSILS. 



23 



illustration. It is well known that these animals, when living, are inva- 

 riably covered with numerous suckers, or gelatinous tubes, called " ambu- 

 lacra!," because they serve as organs of motion. They are also armed with 

 spines supported by rows of tubercles. These last are only seen after the 

 death of the sea-urchin, when the spines have dropped off. In fig. 12 a 

 living species of Spatangus, common on our coast, is represented with 



Fie. 11. 



Serpula attached to 

 a fossil Spatangus 

 from the chalk. 



Eecent Spatangus with the spines 

 removed from one side. 



5. Spine and tubercles, nat. size. 

 a. The same magnified. 



one-half of its shell stripped of the spines. In fig. 11a fossil of the 

 same genus from the white chalk of England shows the naked surface 

 which the individuals of this family exhibit when denuded of their bris- 

 tles. The full-grown Serpula, therefore, which now adheres externally, 

 could not have begun to grow till the Spatangus had died, and the 

 spines were detached. 



Now the series of events here attested by a single fossil may be carried 



a step farther. Thus, for example, we often meet with a sea-urchin in 



the chalk (see fig. 13), which has fixed to it the lower valve of a Crania, 



a genus of bivalve mollusca. The upper valve (b, fig. 



13) is almost invariably wanting, though occasionally 



found in a perfect state of preservation in white chalk 



at some distance. In this case, we see clearly that the 



sea-urchin first lived from youth to age, then died and 



lost its spines, which were carried away. Then the 



ihinus "frorxTthe young- Crania adhered to the bared shell, grew and 



vafve'ofthe'o'awfa perished in its turn ; after which the upper valve was 



attached. separated from the lower before the Echinus became 



o. Upper valve of th3 r 



Crania detached, enveloped m chalky mud. 



It may be well to mention one more illustration of the manner in 

 which single fossils mav sometimes throw light on a former state of 

 things, both in the bed of the ocean and on some adjoining land. We 

 meet with many fragments of wood bored by ship-worms, at various 

 depths in the clay on which London is built. Entire. branches and stems 

 of trees, several feet in length, are sometimes dug out, drilled all over by 

 the holes of these borers, the tubes and shells of the mollusk still re- 

 maining in the cylindrical hollows. In fig. 15 e, a representation is 

 given of a piece of recent wood pierced by the Teredo navalis, or com- 

 mon ship-worm, which destroys wooden piles and ships. When the 

 cylindrical tube d has been extracted from the wood, a shell is seen at 

 ths larger extremity, composed of two pieces, as shown at c. In like 



