256 PATELLIDJE. 



I will now shortly examine tliis condition, and at the same 

 time include the comparison with the Trochi. 



Haliotis, in common with Fissurella, has a double symme- 

 trical branchial plume, two auricles, and a ventricle embracing 

 the rectum, which terminates between the roots of the bran- 

 chiae. The Trochi have not an analogous point : in them, 

 there is only one auricle and one branchial leaf, a heart not in 

 contact with the intestine, and the rectum far removed from 

 it, at the front of the right side. Again, the perforations are 

 for the similar purpose of the fissure in the Fissurellce, for the 

 dejections, and to admit the water to the respiratory vault. 

 Nothing of this obtains in Trochus and further, the fringes 

 and ornaments about the head, and the cordon of filaments in 

 Haliotis are in unison with Fissurella and Emarginula. The 

 Trochi, we admit, exhibit indices on these points, and they 

 are those which show the nearest approach to Haliotis. In 

 addition to these deviations, most malacologists would say, 

 that Fissurella and Haliotis are strict hermaphrodites, and the 

 Trochi bisexual. I was of this opinion, but I believe that there 

 are grave reasons to doubt this arrangement, and which lead 

 me to consider the Trochi subject to one of the modifications 

 of hermaphroditism. These views, and some other very cu- 

 rious circumstances relative to them, will be communicated in 

 our observations on the Trochidae. And lastly, Fissurella and 

 Haliotis are without opercula, the Trochi never ; the former 

 are almost always fixtures, but the latter are locomotive. 



I have now gone through the principal anatomical and 

 external aspects and habitudes of the contrasted objects, and, 

 finding scarcely a point of community between them, I am 

 bound to admit that Haliotis and the Fissurellae cannot be 

 separated, and that the Trochi must rest where we have placed 

 them in our method, in the neighbourhood of their brethren 

 that have circular opercula. 



If the genus Scissurella, now incerta sedis, does not belong 

 to the Trochida, we should not be surprised to find that its 

 position is near Haliotis. 



The Haliotis tuberculata is not strictly British ; it inhabits 

 Guernsey and the other Channel islands. 



