192 



BRITISH CARBONIFEROUS BRACHIOPODA. 



Famify—CRKNWDJE. 

 Genus — Crania, Betzius, 1781. 



The shells composing this remarkable and widely spread genus vary much in shape, 

 although not much difference has taken place in this respect in time, for some Palaeozoic 

 species can hardly be distinguished from more recent and even living types. They are all 

 marginally more or less circular or sub-quadrate, rarely free, but generally attached to 

 marine bodies by the beak (when such does exist) or by the entire surface of the lower 

 or ventral valve ; and it is from this circumstance that the ventral or attached valve varies 

 so much in shape and sculpture. The upper or dorsal valve is always more or less limpet 

 shaped, with a sub-central vertex, the surface being smooth or variously sculptured by 

 concentric or radiating strise, or ribs, some also possessing a spiny investment. There 

 exists no articulated hinge, the valves being kept in place by a peculiar disposition of 

 the muscles ; and although the animal has not been hitherto completely investigated, we 

 will give figures of the interior of the valves, for the sake of explaining the more recent 

 but provisional interpretation and names that have been applied to the muscular 

 impressions by Mr. Hancock. But we must hasten, at the same time, to observe that the 

 interior appearance and shape of the muscular and other impressions are very different 

 in detail in certain species, although very similar in others. The figures here given will, 

 however, suffice to explain the general character. 



Dorsal or free valve. 



PS 

 Ventral or attached valve. 



r. 



v,p 



Crania Ignabergensis (var.), Cretaceous. 



Occlusor (Hancock) = anterior adductors (Woodward). 



Divaricator (H.) = posterior adductor (W.). 



Ventral adjustor (H.). 

 d, p. Dorsal adjustor (II.) = protractor sliding muscle (W.). 

 p, s. [?] Anterior extremity of dorsal adjustor (II.). 



b, p. Brachial muscle, posterior extremity (H.) = retractor sliding muscles (W.). 

 b. Brachial muscle, anterior extremity (II.) = retractor sliding muscles (W.). 

 n. Mesenteric muscle, destined probably to draw the alimentary tube backwards (? H.). 

 o. Ovarian (?), rn. granulated margin. 



