56 BRITISH CRETACEOUS BRACHIOPODA. 



Sand, near Warminster, Cambridge, and other localities, and these have been generally 

 admitted to be the types of T. biplicata} M. D'Orbigny seems, however, to consider the 

 Italian shell specifically distinct from our British one, not from a personal acquaintance 

 with Brocchi's type, but from an assumption of his own ; nor have I been more fortunate 

 in my endeavours to obtain additional information regarding the San Quirico fossil, all we 

 are therefore enabled to judge from, is the figure, and this so nearly agrees with many of 

 the forms found in the Upper Green Sand, near Warminster, that strong doubts may be 

 entertained whether the French author's opinion be really correct. 2 The illustrations 

 furnished by M. D'Orbigny, (Pal. Franc, Terrains Cretaces, vol. iv. pi. 511, figs. 9 — 15,) 

 resemble, in my opinion, the Italian figure far less than some of those of which he forms 

 his Ter. Dutempleana (same work, PI. 511, figs. 1 — 8,) and to this last, he justly refers 

 Sowerby's figures of Ter. biplicata. 



No species varies to a wider extent than the one under consideration, but, at the same 

 time, it does not seem difficult to trace the links which connect by insensible gradation 

 the most extreme variations hitherto observed ; nor am I yet prepared to surrender the 

 opinion expressed in page 53, viz. that passages may not be found similarly connecting 

 these last to the Ter. obesa (Sow.). 



Local conditions have materially influenced the regular development of this, as well as 

 that of other forms ; producing varieties and races, not always easily referable to their 

 original type. Thus, when young, Ter. biplicata is perfectly oval, uniformly convex, and 

 without trace of biplication, but as the animal advances in its development, the dorsal valve 

 becomes more or less prominently biplicated, with a mesial sinus of variable depth and 

 width extending between the plicaD ; but, in some examples, the regular convexity of the 

 dental valve is but slightly influenced by the biplication of the dorsal one. These differ- 

 ences may be observed in the numerous illustrations in PI. VI, and especially in figures 5, 

 9, 17, 36, &c. Much variation is likewise produced in the general contour, by the lesser or 

 greater approximation of the two plains and their ridges, as well as by the different degree 

 of convexity presented by the valves. 



Two principal varieties may be mentioned. 



1. The one abundantly found near Warminster, and in the Isle of Wight, of which 



1 Sowerby mentions the Gault, at Cambridge, as containing his Ter. biplicata, but it would appear that 

 the shell, although common in the clay band of that name at Folkstone and in other localities, at Cambridge 

 is only found in the Upper Green Sand. As justly observed by Mr. Deshayes, many other biplicated 

 species from the oolitic and other formations have been confounded with the cretaceous type : thus Professor 

 Bronn mentions as synonyms of Ter. biplicata, (Index Pal., p. 1230), the following forms which seem all 

 specifically distinct. Ter. bicanaliculata, T. maxillata, T. sella, T. JJarlani, T. bisvffarcinata, and T. pero- 

 valis. V. Buch has likewise erred in several of his supposed synonyms of this species, but was right in 

 placing Ter. faba among them. 



2 In page 53 of this Monograph, published in 1852, I had so far sanctioned M. D'Orbigny's view by 

 admitting the name Ter. Dutempleana ; but subsequent and more complete investigations disposed me to 

 consider that conclusion both premature and uncertain . 



