SAXICAVIDiE. 73 



also towards the other end : ejpidermis membranous : healcs 

 usually separate, not projecting, turned towards the anterior 

 side : ligament external : Mnge strong, furnished with cardinal 

 teeth, which are in some cases small, indistinct, or obliterated, 

 and an upright ledge to support the ligament : ^9a7?/aZ scar 

 placed far inside, and having a broad sinus : muscular scars 

 large and conspicuous. 



A family having close aflfinities with the last^ but 

 different in possessing an external ligament instead of an 

 internal cartilage, and in the consequent structure and 

 apparatus of the hinge. Some burrow in sand or mud 

 like Mya ; others perforate certain rocks and hard 

 substances, to a depth equal to the breadth of the shell 

 and length of the tubes when fully extended. The 

 mode by which these various objects are effected appears 

 to be the same in every case, viz. by the propulsion or 

 attrition of the muscular foot, which is always placed 

 near the posterior end of the shell, assuming when in 

 action the form of a cone or disk, and occupying the 

 space to be excavated. Having already discussed at 

 some length the latter part of the subject in the intro- 

 duction to the first volume, I T\dll not here say more 

 than that occasional notices of this remarkable operation 

 will be found in subsequent pages, while treating of par- 

 ticular genera and species comprised in the Lamarckian 

 group of " Lithophages,^^ as well as of the Teredines. 

 Most of the Suxicavidce pass their lives in a hermit-like 

 seclusion, each immured in its own cell, content with the 

 food brought by the waves or minute currents produced 

 by the siphonal cilia, as well as with a certain degree of 

 immunity from outward foes. Having no means of mutual 

 intercourse, the nature of their sexual organization maybe 

 easily inferred ; and the analogy in this respect between 

 them and many flowering plants, which are rooted to the 

 soil, cannot be very remote. Individuals of the same 



VOL. III. E 



