16 BRITISH CRETACEOUS BRACHIOPODA. 



Plate I, fig. 17. Interior of attached valve magnified. 



,, figs. 18, 19. Exterior and profile of the species magnified, from Gravesend. 



„ figs. 20 — 26. Different specimens from the Chalk of Pewsey, Wilts., 



showing various modes of attachment, from the Collec- 

 tion of Messrs. Cunnington and Faulkner, of Devizes. 



Genus — Argiope, Beslongchamps. 1842. (Megathgris, D'Orb., 1847.) 



Shell inequivalve, variable in shape, semi-orbicular, quadrate, or transversely oval. 

 Valves unequally convex, smooth, or variously costated : the dental valve deepest, beak 

 produced, with a large depressed triangular area, foramen large, completed by the umbo of 

 smaller valve. Structure strongly punctuated ; margin thickened and granulated. Hinge 

 articulating by the means of two teeth in the larger and corresponding sockets in the 

 smaller valve : apophysary system consisting of a distinct loop, originating at the base of 

 the dental sockets, and furnished with converging processes : one or more radiating 

 elevated ribs or dissepiments arising from the inner surface of the smaller valve, the 

 central septum being more produced and larger than the lateral ones, which are often 

 indistinct. 



t Obs. We are only acquainted with one British cretaceous Argiope. (See observations 

 on this genus in Part I.) 



6. Argiope decemcostata, Bcemer. Sp. Plate III, figs. 1 — 13. 



Terebratula decemcostata, Rcemer. Die. Vers. Norddeutschen Kreidgeberges, 1840, 



p. 41, tab. vii, fig. 13. 



— Bronnii, V. Hat/enow. Mori. der. Riigenschen Kreide Versteinerungen 



in Neuv. Jalib., fig.'min. 1842, tab. ix, fig. 7. 



— Buchii, V. Hagenow. lb., tab. ix, fig. 8. 



— Duvalii, Dav. 1847. Lond. Geol. Journal, p. 113, pi. xviii, figs. 15 — 18. 

 Megathiris Cuneiforms, D' Orb. 1847. Pal. Franc. Ter. Cretacees, vol. iv, p. 147, 



pi. 521, figs. 1 — 11. 

 Terebratula decemcostata, Bronn. Index Paleeont., p. 1234, 1848. 

 Megathiris cuneiformis, If Orb. Prodrome, vol. ii, p. 259, 1850. 



Diagnosis. Shell of a somewhat pentagonal transverse form, larger or dental valve 

 convex, beak produced, not recurved, area large and triangular, variable, in its dimensions 

 as wide as the shell; foramen irregularly circular or triangular, occupying often about 

 one third of the area, and laterally distinctly margined by two narrow deltideal plates, 1 the 

 anterior portion being completed by the umbo of smaller or socket valve, which is often 



1 I do not agree -with M. D'Orbigny while stating that this genus is deprived of deltideum ; to me it 

 is quite apparent, although not very great. 



