226 Mr. A. Hancock on the Boring oj the Mollusca into Rocks, 



vol. xv. p. 114, I advanced an opinion that the animal itself is 

 the boring instrument, and that those portions of it which come 

 in contact with the bottom of the excavation are furnished with 

 a peculiar armature for the purpose. But before entering further 

 into this view, it will be better to inquire how far any of the above 

 theories are likely to be correct*. 



In the first place, then, are the excavations effected by the 

 mechanical action of the shell ? The Teredines are the only spe- 

 cies that are stated to bore in the manner of an auger, and are 

 consequently supposed to have extensive rotatory motion. On 

 examination, the perforations of Teredo are found to be nearly 

 cylindrical, perfectly circular, and frequently very tortuous, with 

 their inferior terminations always exactly rounded. The animal, 

 which in T. norvegica, according to Mr. William Thompson 

 (Edin. New Phil. Journ. 1835), is sometimes nearly two and a 

 half feet in length, occupies the whole channel from end to end ; 

 and is stated by Sir Everard Home to be attached to the calca- 

 reous sheath at the siphonal extremity. This attachment is alone 

 sufficient to prevent anything like complete rotation in a con- 

 tinuous direction, and indeed of any rotation at all at the com- 

 mencement of the burrow ; but the difficulty is much augmented 

 when the direction of the bore is taken into account, which, as 

 above stated, is often exceedingly tortuous, turning from side to 

 side in the most abrupt manner, and twisting in every possible 

 direction. It is very common to find the bore turn suddenly at 

 right angles to its original course, and after running a short way 

 to bend again, as suddenly, and return in a parallel direction to 

 the commencement of the track ; thus forming three sides of a 

 parallelogram, the angles at the turns being very perfect. In 

 such a burrow as this it appears quite evident that rotation must 

 be excessively limited, and consequently that the valves cannot 

 cut in the manner of a centre-bit or auger. To this fact almost 

 every piece of wood riddled by the Teredo bears testimony. 



Many of the borers have the anterior portion of the valves 

 covered with spines and raised strise, which has induced the be- 

 lief that these shells are able to rasp down the substances into 

 which they penetrate. Such an opinion, however, cannot be 

 maintained respecting those which have the valves smooth and 

 covered with a decided epidermis, as Saxicava, Lithodomus and 

 Gastrochama : and after a little consideration it will be apparent 

 that the shells with spines and raised strise are likewise inadc- 



* Since drawing up this communication I perceive by the third part of 

 the ' History of British Mollusca,' that the authors of that work adopt Mr. 

 Garner's theory, which however they modify ; the rasping of the valves 

 being considered the principal agent, and the currents produced by the 

 animal only secondary. 



