Mr. J. Lycett on the genus Tancredia. 



223 



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 veiy 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. 



Tancredia donaciformis. 



Sp. char. Shell subtrigonal, transverse, rather depressed, pointed 

 at the extremities ; umbones mesial or antero-mesial, small, 

 depressed ; anterior side attenuated, its superior margin rather 

 concave; posterior side truncated and gaping, its margin 

 straight, posterior to the ligament, and sloping obliquely down- 

 wards; an angle extends obliquely from the umbo to the 

 infero-posterior extremity; ligament short, external, hori- 

 zontal; margins of the valves not close-fitting and rather 

 irregular. 



Upon the principle that our choice of the typical example of 

 a genus should comprise the several peculiarities of the form in 

 a conspicuous manner, I prefer to select the present species to 

 illustrate Tancredia, and will adopt its hinge-characters in the 

 following amended description : — 



Hinge with an obtuse cardinal tooth in each valve, which is 



