ARGIOPE. 17 



worn away at the umbo by friction, presenting thus an accidental false area in the smaller 

 valve; this valve is less convex and more compressed than the dental one. Surface 

 ornamented by a variable number of large rounded costae, between which a slightly concave 

 space is seen ; the two principal ones, or those nearer the centre, are the largest, and more 

 or less separated by a mesial furrow existing in both valves ; the costae and furrows 

 correspond to each other, and do not indent, as in the plaited Spirifers and Rhynchonellas, 

 &c. Valves articulating by means of two teeth in the dental or larger valve, and two 

 sockets in the smaller one; from under the crura in socket valve, a central triangular 

 septum gradually arises, and attains its greatest elevation at about four fifths of the 

 length of the shell, after which it descends by an almost perpendicular line to the bottom 

 of the valve; margin wide all round, and strongly punctuated; in the interior of dental 

 valve, besides the two strong teeth, a small slightly elevated longitudinal septum is seen to 

 extend to about half the length of the shell. Structure punctuated ; dimensions very 

 variable. Sometimes length 2, width 1, and not quite 1 line deep; at other times, length 1, 

 width 1, depth ^ line. 



Obs. This little shell seems to have been first noticed in the Cretaceous Formations by 

 Roemer, under the denomination of Terebratula decemcostata; but the figures of that 

 author not reminding one of the usual form, is probably the reason why the shell has 

 received so many other names, owing likewise to its variable appearance, a character 

 frequently lost sight of by most authors, who, out of varieties, have made many species ; 

 thus, Ter. Bronnii and Buchii of V. Hagenow seem to me, in all likelihood, only varieties 

 with few or more costse ; we frequently find undoubted specimens of this species having 

 either six, eight, or ten ; it is therefore evident, that it is highly improper to give names 

 from such a variable character. Some specimens do not present their greatest width at the 

 hinge line, being rounded in that portion, while others, on the contrary, assume at the 

 hinge the form of elongated wings, giving it a close resemblance to some Spirifers, &c. 

 From my ignorance of Rcemer's and V. Hagenow's figures, I described this species in 1848 

 under the name of Ter. Duvalii, which must be considered a synonym ; a little later, 

 M. D'Orbigny fell into the same error, by proposing that of Cuneiformis ; it is singular 

 that author should have committed this mistake, since, in p. 147 of his ' Pal. Franc.,' 

 vol. iv, while characterising his genus Megathiris, 1 and after having enumerated several 

 recent types, he states • " En especes Fossiles le T. decemcostata of Rcemer ;" and although, 

 admitting this species in his 'Pal. Franc.,' he omits it completely in his 'Prodrome,' vol. ii, 

 1850 ; but allowing even that M. D'Orbigny considered T. decemcostata as specifically 

 different from the chalk species here described, his name would have to give place to 

 one of those published by V. Hagenow in 1842, but with which work M. D'Orbigny was 

 probably unacquainted. 



A. decemcostata bears the greatest outward resemblance to some recent forms of tins 



1 The term Megathiris is a synonym of M. Deslongohamps' genus Aryiope, established long before. 



