On the Shell of Spondylus varius. 407 



Art. II. Observations upon the Camerated Structure in the Valves 

 of the Water-Clam, (Spondylus varius, Sow.,). By Richard 

 Owen, Esq. F.R.S., &c, Hunterian Professor at the Royal Col- 

 lege of Surgeons.* 



Having been led to reflect, while considering the uses of 

 the camerated part of the shell of the Nautilus, upon the de- 

 gree or extent to which that structure might depend upon the 

 mode of growth of the animal and its shell, and how far it was 

 a necessary physical consequence of the increase and change 

 of position of the animal, independently of any special pur- 

 pose served by the forsaken parts or chambers of the shell, I 

 have caused sections to be made of a great variety of shells, 

 and have examined them in the hope of arriving at the law of 

 the multilocular structure, which results physically from the 

 secretion, on the part of the animal, of a nacreous layer, form- 

 ing a new basis of support to the soft parts, and cutting off 

 the deserted portion of the shell from the chamber of occu- 

 pation. 



It is well known that this process is not the only mode 

 adopted to suit the shell to the changing form and bulk or 

 other exigencies of its occupant. In the genus Magilus the 

 part of the shell from w r hich the body gradually recedes is 

 filled up by a continuous compact secretion of calcareous 

 matter, and a solid massive elongated shell is thus produced, 

 which would be a great incumbrance to a locomotive mollusc, 

 but is of no inconvenience to an univalve destined by nature 

 to exist in a fixed and motionless state, buried in a mass of 

 lithophytous coral, with the growth of which it is compelled 

 to keep pace. 



In Helix decollata, again, the deserted part of the shell, 

 after being partitioned off by the nacreous layer secreted by 

 the posterior part of the mantle, is broken away by some yet 

 unexplained process, and consequently no chambers nor any 

 solid apex of the shell remains. 



The retention of the deserted chambers and the intercep- 

 tion of certain spaces of the cell by calcareous septa, though 

 not unknown in the gasteropodous univalves, is more com- 

 mon in bivalves. 



This process is generally consequent on an increase, but 



* The greater portion of the present paper from Prof. Owen appeared in 

 the Proceedings of the Zoological Society for June, 1837, but the illustra- 

 tions and some additional matter have been subsequently introduced, and 

 communicated by the author for publication in the Magazine of Natural 

 History. — Ed. 



Vol. II.— No. 20. n. s p p 



