240 Mr. J. Lycett on the genus Tancredia. 



pressed, oblique, and elongated cardinal tooth in the left valve, 

 the hinge is much altered in its aspect : the variation is exem- 

 plified by several species which occur in our Great Oolite and 

 in the Lias of the Meuse. 



The small accessory cardinal teeth are very uncertain in their 

 distinctness, and constitute only a minor and variable feature. 



The figure of Tancredia varies according as the anterior or 

 posterior sides are the most produced ; several species have the 

 posterior side very short and convex, the figure then nearly 

 resembles that of the recent Donaces : all the species hitherto 

 discovered are destitute of ornament, they are remarkably smooth, 

 and exhibit but indistinctly the lines of growth. 



The number of species now known afford sufficient data for 

 comparison with other genera, and to determine its position in 

 the malacological system. To existing genera it would appear to 

 be only remotely connected ; but there are certain fossil forms, as 

 yet insufficiently known, which seem to approach to it in several 

 particulars ; but whether these latter forms, which are associated 

 with Tancredia in the same beds, are entitled to a position distinct 

 from existing genera, remains to be determined. M. Buvignier 

 has only indicated the position of Hettangia by placing it with 

 the Cardiacese. 



