RHYNCHONELLA. 81 



Rhynchonella compressa, IfOrbiyny. Pal. Franc;., Ter. Cretaces, vol. iv, p. 35, 



pi. 497, figs. 1—6, 1847. 

 — difformis, IfOrbigny. lb., vol. iv, p. 41, pi. 498, figs, 6 — 9, 1847. 



Terebratula C0MPRESSA,.B;wm. Index Pal., p. 1233, 1848, (but not a Syn. oiT.limbata.) 



Diagnosis. Shell depressed, elongated oval, wider than long, angular at the cardinal, 

 dilated towards the pallial, region, somewhat indented in front ; the greatest width and 

 depth lying towards the middle of the shell: valves unequally convex, the dorsal one gene- 

 rally the deepest, with a wide, slightly produced, and flattened mesial fold, occupying 

 about one third of the width of the shell : in the ventral valve, a corresponding wide 

 longitudinal sinus : beak acute, moderately produced, and incurved : foramen rather small, 

 and entirely surrounded by the deltidium : beak ridges sharply defined, leaving a 

 flattened space between them and the hinge line : externally each valve is ornamented 

 by from 32 to 48 strong simple plaits, 8 to 11 of which compose the mesial fold and 

 sinus. 



Dimensions variable: length 17, width 23, depth 8 lines. 



>> 13, ,, 18, „ 10 „ 



Obs. This fine species was described by Lamarck, in 1819, from specimens derived 

 from the Upper Green Sand of Mans (France) : it varies greatly in degree of compression, 

 some examples being considerably flattened, while others are more convex, and this last 

 variety is the one commonly found both at Chute, near Warminster, and Cap-la-Heve, 

 near Havre (France). A similar shell was described at a later period (1836) by Sowerby, 

 under the name of Ter. dilatata, and Ter. Gallina (Brongniart) seems likewise to belong 

 to the same type. B. compressa is not always regularly trilobed, but often unsym- 

 metrical, from the mesial fold becoming totally or partially shifted either to the one or 

 other side ; the shell then appears divided, as in Bh. inconstans, into two portions, one half 

 occupying a higher level than the other, or with one edge turned up and the other down ; 

 a malformation so common among the Bhynchonellce that it cannot be made use of as a 

 character of any specific importance : thus Terebratula difformis 1 (Lamarck), and Ter. 

 dimidiata (Sowerby), are nothing more than irregularly developed examples of B. com- 

 pressa, of which any one will become convinced who may examine the typical specimens in 



1 M. D'Orbigny seems to consider Rhyn. difformis (Lamarck) to be specifically distinct from 

 R. compressa of the same autbor, and states p. 42, vol. iv, of the 'Pal. Franc.,' "Cette espece (R. dif- 

 formis) se distingue du T. contorta par ces cotes plus grosses. Lorsqu'elle est reguliere, elle se rapproche 

 du R. compressa, mais elle differe par sa forme plus renflee encore, est plus courte, moins dilate'e laterale- 

 ment, c'est une espece bien separe'e, mais tres variable dans sa forme." Lamarck observes, that 

 his specimens of this shell were derived from the Green Sand of Cap-la-Heve (near Havre), and likewise 

 from Mans. And in both localities I have had the opportunity of examining and collecting specimens, 

 uniting these malformations by insensible passages to the regularly developed condition of R. compressa ; 

 and both in the French and British localities we find unsymmetrical individuals likewise more or less 

 flattened, as is the case with well-shaped examples. 



