254 Mr. T. Davidson's Notes and Descriptions 



the margin. A sinus corresponds to the fold of the smaller valve, 

 the perforated valve being rather less convex than the imperfo- 

 rated one ; beak not much produced, recurved and acute, under 

 which is seen a small circular foramen entirely surrounded and 

 separated from the umbo by a deltidium in two pieces ; beak- 

 ridges well defined, leaving a slightly concave space or false area 

 between them and the hinge-line, which last slightly indents the 

 smaller valve ; surface ornamented by a great number of small 

 plaits, from fifty to sixty on each valve. Structure unpunctuated. 

 Length 23, width 25, depth 14 lines. 



Obs, This fine Rkynchonella was discovered by M. Deslong- 

 champs in the liasic beds of Fontaine-etoupe-four near Caen, 

 where it seems rare. M. Tesson has also obtained it in the same 

 locality; it differs from R. trilohata, Miinster, by its much 

 smaller and more numerous plaits ; this last being also an Upper 

 Jurassic species, while our present form is liasic. The specimen 

 figured is from the collection of M. Deslongchamps, to whom I 

 have the pleasure of dedicating it. 



Rkynchonella Boueti, Dav. 1852. PL XIII. fig. 4, 5* 



Diagnosis. Shell inequivalve, wider than long, irregularly tri- 

 angular and trilobed, the smaller valve being almost equally di- 

 vided into three longitudinal portions, the central one forming an 

 elevated mesial fold laterally contracted with a deep correspond- 

 ing sinus in the larger valve, the front forming three almost equal 

 and convex projections ; beak not much produced, acute ; fora- 

 men small and entirely surrounded and separated from the umbo 

 by a deltidium in two pieces ; beak-ridges well defined, leaving a 

 slightly concave false area between it and the hinge-line, which 

 last does not greatly indent the imperforated valve ; surface or- 

 namented by a variable number of simple acute plaits, about 

 thirty in each valve — three, four or five forming the mesial fold 

 and sinus ; margin thickened all round. Structure imperforated. 

 Length 12, width 15, depth 11 lines. 



Obs. This shell occurs along with R. obsoleta in the Great 

 Oolite of Ranville near Caen, where it is very common, and is 

 easily distinguished from the last-named species, particularly by 

 its much more elevated mesial fold gradually arising from the 

 umbo till it acquires its greatest elevation near the front, while 

 in i?. obsoleta it will be found to exist at about the middle of the 

 shell, the remaining portion sloping down to the front ; the 

 lateral parts are likewise more elongated and somewhat wing- 

 shaped, a character not peculiar to obsoleta, which has a more 

 simple and circular form. 



