COBELLA. 19 



ated that we deemed it right to make the change. 

 Since then two other very interesting species have 

 been discovered which agree in all essential characters 

 with this type ; thus in a measure justifying the 

 establishment of the genus, the members of which are 

 all sinistral. The alimentary tube is very differently 

 disposed from that of Ascll'm ; the heart occupies a 

 different position, and the spiral arrangement of tlie 

 secondary branchial vessels seems alone sufficient to 

 distinguish the genus. 



As the internal organization is very similar in all the 

 three species, we will confine the following description 

 of it to that of G. parallelogramma, which we consider 

 the type form. This animal is scarcely more remark- 

 able for its internal organization than it is for the 

 brilliant colouring of its mantle, and for the smoothness, 

 transparency, and glossiness of its test, which is thin 

 and only slightly, if at all, contractile, but possesses 

 considerable elasticitv. 



tj 



The inner tunic or mantle is delicate, thin, and 

 transparent, though usually highly coloured. It is 

 deficiently supplied with muscular fibres, which are 

 arranged, for the most part, near the margin in a 

 radiating manner, the bundles being slender and far 

 apart. This is the case with the right side ; the left 

 is almost deprived of muscles, there being only a few 

 scattered, delicate fibres at the base of the inhalant 

 tube, placed longitudinally. The respiratory tubes 

 themselves are sparingly supplied with a few feeble, 

 delicate, circular and longitudinal fibres. 



The branchial sac (fig. 30) is comparatively short, 

 and is sub-quadrate in form. It reaches to the bottom 

 of the pallial chamber, the posterior extremity passing 

 on the right of the digestive organs. The whole 

 aerating surface presents a rather coarse rectangular 

 reticulation, formed by the primary transverse vessels, 

 and containing almost equally stout, longitudinal secon- 

 dary vessels. The square meshes so formed are in 

 transverse series, and are filled up with pretty-regular 



