18 . BRITISH CRETACEOUS BRACHIOPODA. 



genus found in the Mediterranean ; but in the interior of these last, the three well-defined, 

 elevated septa, figs. 14 and lG ab of our Plate, are prominent, which is not the case 

 in the interiors of the chalk species I have seen, and which, I must confess, I have never 

 observed in the Meudon specimens, as represented in figs. 9 and 10 of M. D'Orbigny's 

 Plate 521, the central one only being perceptible. 



This remarkable variation presented by the fossil species above described, is likewise 

 equally prevalent in the small recent type originally described under the appellation of 

 Anomia decollata, by Chemnitz, 1 as I became convinced from a numerous suit of this 

 species given to me by Professor Forbes, and dredged by himself in the Mediterranean, 

 these variations having tempted Risso 2 to propose the following names : — Ter. urna-antiqua, 

 T. cardita, T. emarginata, T. cuneata ; some of which being likewise adopted later by 

 Sig. Philippi 3 and others, proving how difficult it is often to pronounce with certainty the 

 limits to be assigned to a species. 



In many adult specimens of A. decemcostata, as already noticed, the umbo is much worn, 

 no doubt from the shortness of the muscular pedicle of attachment forcing the shell to lie 

 so close to the object to which it is fixed as to wear it by continual friction, caused by the 

 opening and shutting of the valve ; this is likewise the case with many specimens of the 

 recent A. decollata, as is seen from the fig. 16 of our Plate; but in young specimens, and 

 even in more aged shells, the pedicle fibres must have been longer, as both beak, area, and 

 umbo are quite perfect, showing no traces of the friction above alluded to. 



Argiope decemcostata is found in the Upper Chalk of Gravesend or Northfleet, and 

 occurs more abundantly in the Chalk filling Echinodermata, in the washing of which 

 Mr. Purdue procured many specimens. Mr. Harris has likewise met with it in the Chalk 

 detritus of Charing (Kent) ; Mr. Cunnington discovered it at Pewsey, in Wiltshire, and 

 is often found in company with Th. Wetherellii. On the Continent it is not very rare at 

 Meudon and other places, although always troublesome to obtain, from its extreme 

 minuteness. M. V. Hagenow found it also along with other interesting forms in the Chalk 

 of the Prussian or Belgian dominions. 



Plate III, figs. 1, 2, 3, 4, 5. A specimen from the Chalk detritus of Charing, Kent, 



in the Collection of Mr. Harris. 



„ fig. 6. Interior of smaller valve magnified. 



,, fig. 6 a . Section of smaller valve magnified, a central septum. 



„ fig. 7. Interior of larger or dental valve enlarged. 



„ figs. 8, 9. A short square variation, with few costae, from Gravesend. 



,, fig. 10. Idem. 



„ figs. 11, 12. A very transverse spirifer-shaped variation. 



1 ' Systematisclie Konchylien Habitat,' vol. viii, pi. lxxviii, fig. 705, 1/85. 



2 'Hist. Nat. des Principales Prod, de l'Europe Meridionale,' vol. iv, Nos. 175, 177, 179, 180, 1826. 



3 ' Enumeratio Moluscorum Siciliae,' 1836. 



