raised above the lowest level of the valve posteriorly, and is bounded 

 anteriorly by a moderately prominent, transverse, rounded ridge, 

 which is cui'ved shallowly towards the front margin, in the middle, or 

 bent towards the front at a very obtuse and rounded angle. The mus- 

 cular impressions are not distinguishable. 



Length of the most perfect specimen along the median line, fifty- 

 eight millimetres : maximum breadth, forty-three mm. : greatest 

 thickness through the closed valves, forty-five mm. 



Durham, Mr. J. Townsend : one perfect specimen, with the valves 

 slightly displaced, and four detached ventral valves. Two of these 

 separated valves have the interior completely filled with the matrix, 

 and the others are so much worn or eroded inside that some of the 

 characters of the interior of the ventral valve cannot be satisfactorily 

 ascertained. 



In all the previously described species of Mmomerella the ventral 

 valve is more or less flattened, its umbo and beak are erect, and its 

 hinge area is distinctly triangular. The ventral valve of the pre- 

 sent species, on the contrary, is remarkably tumid and inflated, its 

 umbo is prominently arched, its beak incurved, and its cardinal area 

 crescentic in outlLne. When its valves are closed M. ovata looks not at 

 all unlike a Pentamerus of the type of P. obbngus or a large Meristella 

 but the internal character of its ventral or pedicle valve seem to show 

 that it is a true Monomerella. ' 



MONOMEEELLA OVATA, Var. LATA. 



Plate 2, figs. 2 & 2o, & plate 8, figs. 2 & 2a. 



Ventral valve (the only one known at present) moderately convex 

 with or without a mesial depression : outline sub-circular : length and 

 breadth about equal : umbo somewhat prominent, beak slightly in- 

 curved : surface concentrically striated : test thick. 



Hinge area concavely arched in front, obscurely sub-angular in the 

 centre behind : umbo double chambered : umboual cavities wide and 

 deep: lateral muscular scars of the platform rather large, rhombic 

 ovate, longitudinally striated, and converging anteriorly but without 

 meeting. Other characters as in the type of the species. 



Durham, Mr. J. Townsend : two ventral valves with the test pre- 

 served, and a well preserved natural cast of the same valve. 



The best specimens of all the species of Trimerellidae which are de- 

 scribed in the present paper have been sent for examination to Thomas 

 Davidson, Esq., F.E.S., to whom the writer is indebted for valuable 



