238 Mr. J. Lycett on the genus Tancredia. 



tional species have been ascertained in our Great Oolite, and the 

 hinge- characters of a fine Inferior Oolite species have been deve- 

 loped : as the latter shell, from its superior size and the promi- 

 nence of its dentition, constitutes a remarkable example of the 

 genus, I propose to describe it in detail, premising that the same 

 species, in a greatly diminished form, was figured in the plate 

 which accompanied the memoir of 1850, under the name of 

 T. donaciformis. The small figure there given represents the 

 usual size of specimens obtained in the shelly freestone of Leck- 

 hampton Hill ; the larger examples now to be described occur not 

 unfrequently in the bed called Gryphite grit, at Rodborough Hill, 

 near Stroud, a locality which has produced so many novel and 

 finely-preserved testacea. Upon comparing the hinge of the 

 new shell with that of T. extensa, which was figured in my me- 

 moir, the difference between them is found to be considerable, 

 and it requires a close scrutiny to perceive that the parts and 

 their arrangement are alike in both, modified by the more ad- 

 vanced growth of the larger shell, and still more so by the pecu- 

 liarities of the species. 



The dental characters of T. externa have much less prominence ; 

 they project but little vertically, and are more extended longi- 

 tudinally. A similar difference is observable between the species 

 which M. Buvignier has figured : his T. Broliensis in its hinge 

 approaches to that of our large shell ; but the hinge of his smaller 

 and more elongated species, Deshayesea, presents a near resem- 

 blance to that of our T. extensa ; the greater obliquity of the 

 cardinal tooth in the elongated species is strongly marked, more 

 especially in the left valve. The Rodborough examples of T. do- 

 naciformis may be regarded as representing the hinge-features 

 in an exaggerated form, the result in some degree of greater age, 

 inasmuch as smaller shells from the same locality lose much of 

 this prominence of character. There would seem to exist much 

 variability in the margins of the valves : all the specimens figured 

 by M. Buvignier have a considerable aperture at the truncated 

 posterior border ; our Cotteswold examples present this character 

 much modified ; it is however very evident in T. donaciformis ; 

 but, strictly speaking, the borders of the valves are not close- 

 fitting along their extent. 



The fact that four, and perhaps five, species of Tancredia have 

 been obtained in the Lias of France, will, it is trusted, induce 

 collectors to examine the same formation in Gloucestershire with 

 increased attention. M. Buvignier does not record the genus 

 in the Oolites. In England it has hitherto been recognised only 

 in the lower oolitic system, which would appear, from the work 

 of M. Buvignier, to be very partially and inadequately repre- 

 sented in the Department of the Meuse. 



