RHYNCHONELLA. 83 



Rhynchonella lata, lybrbigny. Pal. Franc., Ter. Cretaces, vol. iv, p. 21, 1847 (but 



perhaps not all his list of Synonymes). 

 Terebbatvjla plicatilis, Bronn. Index Pal., p. 1246, 1848 (but not T. plicatilis, 



Sow., nor the generality of Bronn's other synonymes). 



— lata, Austen. Quart. Journ. Geol. Soc, vol. vi, p. 4/7, 1850. 



— latissima, Sharpe. Quart. Journ. of the Geol. Soc., vol. x, p. 192, 1853. 



Diagnosis. Shell transversely oval or unequally five sided, with rounded angles .- 

 ventral or dental valve moderately convex : beak acute, slightly produced and incurved : 

 foramen circular, entirely surrounded by the deltidium and its tubular projections : beak 

 ridges well defined, leaving a flattened space between them and the hinge line : the regular 

 convexity of the valve is interrupted by a sinus of moderate depth, commencing towards 

 the middle and extending to the front. The dorsal or socket valve is either convex and 

 regularly arched, or somewhat flattened, with a mesial fold not rising much above the 

 uniform convexity of the shell. Externally, the surface of each valve is ornamented by 

 from 50 to 80 plaits. 



Length 12, width 16, depth 7 lilies ; 

 1 Oi 19 7 



9, „ 10, „ 6 „ &c. 

 Obs. The shells here described may perhaps only constitute a variety of R. compressa, 

 Lamarck : but they seem to be distinguished by a less expanded, and in general more 

 regularly transverse oval shape ; also by the number and quality of their plaits, which are 

 more numerous in the shells under notice than in the Lamarckian type, which does not 

 appear to present the complex condition at times observable in B. latissima (Sow.), 

 So much so, that some examples illustrated in my PI. XI, figs. 19 — 22, have been by some 

 authors supposed to belong to another species, viz., M. antidichotoma of Buvignier, 1 

 but after having examined a numerous series of specimens collected by Messrs. Sharpe, 

 Lowe, Waterhouse, Cunnington, myself, and others, I was able to convince myself in a 

 most satisfactory manner, that all the examples illustrated in PL XI, figs. 6 — 22, belonged 

 to the same species. In the extensive series principally derived from Warminster 

 and Farringdon, every possible variation in the plication may be perceived : in almost all, 

 the plaits are few in number in the young, but soon augment at variable distances from 

 the extremities of the beak and umbo by the intercalation of a fresh plait between those 

 already formed. In many examples, the last as well as the original ones proceed 

 uninterruptedly to the margin, while in others, some of the intercalated ribs are lost, or 

 disappear between their immediate neighbours before reaching the front or margin, while 

 in some cases only a few while in other examples almost every two of the plaits unite, and 

 form a belt of larger costse near the front and margin. All these complex characters are accu- 



1 This shell differs from R. latissima by its general shape and small foramen, which is widely separated 

 from the hinge line by a largely developed deltidium ; also to some extent by the character of its plaits. See 

 Pal. Franc. Terrains Cretaces, vol. iv, p. 500, fig. 1 — 4. 



