410 On the Structure of the Shell 



fore, from the conditions under which the partitions are suc- 

 cessively secreted, that they must adhere together, not only 

 at their circumference, which forms the outer wall of the valve, 

 but to the preceding and succeeding septum, at the part occu- 

 pied by the adductor muscle, and for an extent corresponding 

 to its circumference. The progressive change in the position 

 of this muscle by the absorption of the posterior fibres, and 

 the addition of others anteriorly, changes in a corresponding 

 degree the relative position of these sub-central confluent 

 parts of the septa, and a beautiful undulated disposition of the 

 whole chambered part results. If the adductor muscle were 

 a tube instead of a solid mass, the central confluent part of 

 the septa would of course be perforated, and a siphon would 

 result, the calcareous walls of which, from the proximity of the 

 chambers, would no doubt be continuous, as in many fossil 

 Polythalamous shells. 



A disposition to form chambers is manifested, but in a 

 much less degree, in the smaller flattened or superior valve of 

 the Spondylus varius. In the specimen here figured there 

 are three chambers, with narrower intervals, and much thick- 

 er partitions than in the lower valve. These partitions are 

 confluent opposite the muscular impression, as in the lower 

 valve, and each partition expands from this attachment in an 

 infundibular manner, which reminds one of the emboitement 

 of the calcareous parts of the siphon in the Spirula. 



The secreting power of the lower lobe of the mantle in 

 the Spondylus seems to be greater than in the upper ; it is 

 certain at least that the abundant deposition of calcareous 

 matter upon the margin which forms the hinge of the shell, 

 and which is due to the part of the mantle extended behind 

 and between the cardinal teeth, takes place almost exclusive- 

 ly in the lower valve : and thus the layers of nacre which are 

 successively deposited on the cardinal margin, push forward 

 in a corresponding degree the upper valve, leaving a heel or 

 umbo behind the hinge of the lower valve. But this irregu- 

 larity in the results of the secreting faculty is not attributable, 

 as I once conceived,* to an inherent defect of power in the 

 upper lobe of the mantle, but simply to the circumstance of 

 the corresponding valve being free. If it were fixed by its 

 posterior margin to a foreign body, as in the lower valve, and 

 the same stimulus of necessity were consequently to affect 

 the calcifying organ of the upper valve, it is highly probable 

 that it would manifest the same activity as that of the lower 

 valve. 



*' Proceedings of the Zool. Soe.' June, 1837. 



