368 Mr. Robert Garner's Malacoloyical Notes. 



tive of but one valve of the bivalve, and not of two, as we 

 think is the case with Patella and Fissurella. In Dolabella 

 one valve is developed, the other all but gone. 



Perhaps the normal position of the vent in Gastropods, 

 where there is no flexure super se, is, as in most other 

 animals, towards the opposite extremity to the mouth ; 0>i- 

 chidium, Doris^ and Chiton (representatives of the pulraonate, 

 naked-gilled, and cover-gilled Gastropods) are examples of 

 this : but, as said before, the branchi^ and their accompanying 

 heart are in especial relation with the shell ; and as this deve- 

 lops, the tendency is for the concomitant removal of branchias 

 and vent forwards and to the right (Pleurobranchiata), or 

 especially forwards (Prosobranchiata). This transference may 

 be seen to take place progressively, in the Pulraonata, in On- 

 chidium^ Testacelluj Limax^ andHeliw^ in the Nudibranchiata, 

 in DoriSj Eolis, and Pleurohranchus, in the Cyclobranchiata 

 and Scutibranchiata, in Chiton, Microschisma, Fissurella, 

 Emarginula, Gadinia, and Natica — a remarkable transposi- 

 tion of parts ! 



The shell, then, of the Patella corresponds to the two con- 

 joined valves of a lamellibranchiate bivalve ; they are less 

 conjoined in Fissurella, Emarginula, and Haliotis, in Avhich 

 last is perhaps seen, as already observed, the division into 

 right and left valves in the row of foramina and the long 

 fissure of the mantle. The spiral form becomes progressively 

 more pronounced in Grepidida, Calyptrcea, and Sigaretus ; 

 and this is accompanied by a want of development on one 

 side affecting heart, branchiae, and other organs. Indeed, with 

 respect to the spiral univalves the preceding paragraph re- 

 quires some qualification ; for in them the torsion of the body 

 and the great development of one side (the right) causes the 

 frequently single branchial tuft to be carried to the left, accom- 

 panied by the heart. 



We have always felt disposed to receive the theory held by 

 Adanson, Oken, Dr. Gray, and Macdouald, with respect to 

 the normal spiral Gastropod. The latter writer shows the 

 correspondence and resemblance between the operculum and 

 the univalve shell. He looks upon the muscles connecting 

 the operculum with the spiral shell as the adductors of the 

 dimyary bivalve, and perhaps also combining the representa- 

 tives of those fibres which serve to retract the foot. He sup- 

 poses the body of the bivalve, now occupying the left valve, 

 to revolve from left to right on its longitudinal and transverse 

 axis, in both cases moving through a quarter of a circle. 

 There is no violence done to fact here ; the greatest requisition 

 demanded appears to be the flexure or displacement forwards 



