55*2 MODIOLI ENCLOSED IN LTTHODOMI. 



lustration, and will briefly subjoin such observations as I have 

 made, with a view to explain the appearances they present. 



69 



a, the Lithodomus containing one or more specimens of Modiola. b, the opposite 



side of the same specimen, but with the external shell broken away, so as to show one of the 

 contained Modiolce. c, a Lithodomus in which, from the gaping of the valves of the 



inclosed Modiolce, three or four individuals may be distinguished. 



The size of the figures is eniarged by half a diameter. 



1. It will be observed in the specimen (fig. 69 a) that the 

 outer shell is extremely different from that which it contains 

 (see b). Now although I have repeatedly detected a similar 

 arrangement — the outer smooth shell (Lithodoinus) with its 

 strongly-marked lines of growth containing, and the sharp, 

 angulated, reticulated shell (Modiola?) being contained — yet 

 I never met with an instance in which this order was reversed. 

 This I conceive to be a particular of some importance. 



2. Among the many specimens that have come under my 

 observation, I have never seen a single instance in which the 

 contained Modiola (?) could be distinctly shown to be a bor- 

 ing shell. Even when it appears to occupy a perforation by 

 itself, the difference in size between the hole in which it is 

 situated and itself, and sometimes other circumstances addi- 

 tional to this, seem to show that it is merely the inhabitant 

 but not the fabricator of the orifice in which it has existed. 



3. In cases in which there are more than one contained 

 shell (as shown by fig. 69. c), the additional ones are, I be- 

 lieve, uniformly of the same species with the first- contained 

 shell, which is constantly a Modiola and never a Lithodomus. 



4. Although these shells are almost invariably found enve- 

 loping one another like a nest of pill-boxes, yet I have in one 

 instance seen two small ones placed endwise, the one towards 

 the other, filling up the cavity of a much larger Lithodomus. 



After what I have said it is scarcely necessary to add that 

 I consider the contained shell a true Modiola, and conse- 

 quently not a boring animal : — that it occupied the cavities 

 formed in the coral by the Lithodomi, and very frequently 

 filled the unoccupied shells of the Lithodomi themselves. — 

 But although it might be supposed that one Modiola when 



