78 BRITISH CRETACEOUS BRACHIOPODA. 



descriptions and figures of Ter. plicatilis and T. octoplicata are so entirely similar, that no 

 one is able to perceive in them distinguishing features. R. plicatilis varies to a consider- 

 able extent, as do all Brachiopoda, and it is not unusual to meet perfectly adult individuals 

 of various dimensions as well as convexity, due, no doubt, to more or less favorable condi- 

 tions of existence. After much examination I entertained a similar opinion to that already 

 expressed by several Palaeontologists, viz., that those examples in which, at a certain age, 

 the plaits became united two by two near the front and literal margins, could only 

 constitute a simple variety of Rh. plicatilis, especially as a similar tendency is common to 

 individuals of various forms, as for instance, 11. latissima, Sow., &c. This complex 

 plication does not always take place only in those examples in which the plaits are acute 

 to the very edge. Nor do all specimens of t- ue Rh. plicatilis present, the split condition 

 of the plicae above described, although such may he the prevalent character in most 

 examples. The term octoplicata is in itself essentially ill-chosen, from the positive fact 

 that nothing is more variable than the number of plaits, and as an illustration of which I 

 have figured in PI. X a series of examples collected in the same locality by Mr. Fitch, 

 with 3, 5, 6, 7, 8, and 9 plaits on the mesial fold, and specimens with exactly 8 plaits are 

 by no means the most abundant. Although I have not yet obtained in England examples 

 of R. plicatilis quite as large as some of its variety octiplicata, still in other countries 

 typical specimens of R. plicatilis have been found equalling in dimensions any of those of 

 the variety (PI. X,'figs. 1 and 3). 



I must, however, here observe that Mr. S. P. Woodward differs from the conclusions we 

 have arrived at, and is of opinion that R. octoplicata can be distinguished and should be 

 preserved as a seperate species from R. plicatilis. Associated with the shell last mentioned 

 we often find another form, figured in 1833 by Woodward as Tereb. Gallina (Brong.), 1 but 

 which, although somewhat similar in external contour to the Sowerby species, appears to 

 possess a facies of its own, and, if not specifically distinct from R. plicatilis, would, at any 

 rate, constitute a well-marked variety, being distinguished by fewer plaits, which are pro- 

 portionally wide, with flattened ridges, and usually split near the margins. 



Plate X, figs. 37 to 39. Typical example of R. plicatilis, from the Chalk of Brighton. 



,, fig. 40. Eront view of a very inflated individual, from the Kentish Chalk, in 



the collection of Mr. Bowerbank. 



„ figs. 41, 42. An unusually expanded var., likewise from the Kentish Chalk. 



„ figs. 1 to 11. A series of examples of the var. octoplicata (of authors), from 



the Norwich Chalk, in the collection of Mr. Pitch ; figs. 1 

 and 3 are the largest British specimens I have seen. 



,, figs. 12, 13. A young flattened example, from the Chalk at Royston (Cam- 

 bridgeshire), in the British Museum. 



,, fig. 16. A specimen, exhibiting spots attributed to colour? from the Chalk 



of Norwich. 

 1 ' An Outline of the Geol. of Norfolk, pi. vi, fig. 12.— See likewise our PI. X, figs. 43 to 46. 



