TO A. HANCOCK, ESQ. 267 



genus, and find these three facts occurring in three different 

 species from different quarters of the world. 



My object in writing is to ask you to look to one point in your 

 collection ; but first I will mention what results I have come to. 

 I began with a very strong leaning to the view which you advo- 

 cate, that the excavation must be due to mechanical agency, but 

 unwillingly I have been driven to hypothetical chemical action. 

 My grounds of belief are as follows, and I should be grateful for 

 your opinion, viz. : — 



(1) I can discover no sort of boring contrivance on margin of 

 shell, or on under side of basal membrane ; and there is no differ- 

 ence in appearance in these parts when an individual has bored 

 and has not in the least bored. I have examined the single shell, 

 and cleaned with potash, and after acid, with all powers. 



(2nd) Either the shell or basal membrane must, on mechanical 

 theory, be the wearing agent; and certainly, as far as the central 

 hollow, it must be the basal membrane ; but the basal membrane 

 is united to the shell and animal's body by (besides corium and 

 epidermis) only a circle of fibres, which Prof. Quekett, after 

 most careful testing, says are only ligament: hence I think it 

 impossible that the basal membrane can be moved (at least near 

 the circumference, where the animal's cirri cannot reach), or, 

 again, that the shell can be moved, if we look at the basal mem- 

 brane as the fixed point. 



(3rd) When a central hollow has been formed, the basal mem- 

 brane (in this case generally brittle or cracTced) is loose over this 

 middle part, but was once certainly attached, as I have found 

 the prehensile larval antennge in the middle surrounded by the 

 ordinary cirripedial cement, which certainly would require con- 

 siderable mechanical power to separate from any object of at- 

 tachment, and yet there is nothing whatever over this central 

 portion of the basis but the open sack : dissolution of the shell, 

 on the other hand, to which the cement was attached, would 

 perfectly explain the appearance. 



(4th) As you state the epidermis of shells quite prevents the 

 wearing, except where abraded or cracked ; and I further find 



