56 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



4. The upper valve has a different structure from the lower. 



5. Each valve is unsymmetrical. 



6. The valves are right and left, — not dorsal and ventral. 



7. They are articulated by teeth and sockets, — which is not the 



ease with Crania ; and the teeth are developed from the 

 free valve ; — in all hinged Brachiopods the teeth are in the 

 fixed valve. 



8. The HippuritidcB had a large internal ligament (like Spon- 



di/lus) for opening the valves. 



9. The muscular impressions are two only. 



10. The so-called "vascular impressions " are on the rim of the 



shell, not in the disk, as in Crania^ &c. 



1 1 . The Hijjpurites have a distinct pallial line. 



VII. M. D'Orbigny also regards the Hippuritidce (including Re- 

 quienia) as Brachiopoda ; but he does not place them with the 

 normal Families, or even with Crania. In his latest work, the Cours 

 elementaire (1849), p. 90, he proposes to associate them with the 

 still-existing genera Argiope and Thecidium, under the term " Bra- 

 chiopodes cirrhides : Cirrhidae," with the rank of a Sub-order ; and 

 describes them as having no oral arms, but a mantle fringed with 

 long cirri, performing the f miction of the brachia. 



When Prof. E. Forbes returned from the ^gean, he furnished me 

 with specimens of Argiope decollata, and wdth a sketch, from me- 

 mory, of the oral arms of A. cuneata. In some of these specimens I 

 detected a well-developed loop, and in others the animal itself. Mr. 

 Davidson, who examined them with me, has published our drawings 

 and notes in the ** Introduction " to his Monograph. There is no 

 doubt that Argiope is very nearly allied to Terebratula, the differences 

 having reference chiefly to the minute size of all the species. Argiope 

 has cirrated oral arms, supported by a distinct loop ; the mantle- 

 margin is quite simple. We could not ever detect the presence of 

 set(S, such as exist in Terebratula and Rhynchonella. 



We have also seen examples of the recent Thecidium mediter- 

 raneum, which differs from Terebratula chiefly in being fixed by 

 the ventral valve, and not by a pedicle. It has a developed loop, 

 supporting cirrated oral arms, and can only be regarded as an aberrant 

 member of the family Terebratulidce. The mantle-margin is quite 

 simple. 



VIII. Those authors who have regarded the Hippurites as Bi- 

 valves, forming a distinct Order (Rudista), intermediate between the 

 Pallio- and Lamelli-branchiata, viz. Lamarck, Blainville, and Rang. 



M. Sander Rang adopted this opinion on account of the diflSculty 

 of reconciling the supposed characters of the Rudista with the known 

 characters of the ordinary Conchifera. He could not account for the 

 two holes in the lid of the Hippurite, and the ridges {aretes) inside 

 its lower valve. 



The "holes " in the upper valve are only found in a few species 

 (H. bi-oculatus and H. dilatatus, Defr.) ; they are mere depressions, 

 — points at which the lid rests on the two posterior inflections of the 

 lower valve. 



