59 



impressions can rarely be aseertained, or even tlic true surface markings. 

 As tlie whole of the above remarks apply with full force to the present 

 fossil, it may be easily imagined that its generic position is doubtful. 

 Further, the only specimen yet collected has had its original shape so 

 much altered by compression that the specific description may have to 

 be materially modified or altered, when better examples have been 

 obtained. 



PnOLADOMYA OVULOIDES. (N. Sp.) 

 Plate IX, figure 9. 



Shell swollen and ventricose in front, rapidly decieasing in thickness 

 behind ; height more than one-third less than the length ; outline sub- 

 ovate. The superior border is straight and nearly horizontal, if viewed 

 laterall}', but as seen from above it is concavely inflected on each side, and 

 the result is that there is a well-defined, narrowly lanceolate, excavated 

 escutcheon. The inflection is so decided as to present the a])pearance of 

 an obtuse ridge on each valve, and both of these extentl in a slightl}^ con- 

 vex curve from behind the umbones to the posterior end of the hinge line, 

 which is sunk below their level. The umbonal region is much inflated, 

 but the beaks themselves are not very large, and do not project much 

 above the hinge margin; they are situated very near the anterior ex- 

 tremity, but are not quite terminal ; their aj)ices are incurved, approx- 

 imating, and point very slightly forwards. In front of, but just under 

 the beaks, the hinge line is short, straight, and oblique, with a distinctly 

 downward slope ; there is no lunule. The anterior prolongation of the 

 hinge line is mostly concealed by the upward swell of the beaks, so that 

 in some aspects there appears to be a concave lunular declivity. The 

 anterior end is angular a little above the middle, subtruncate in the centre, 

 and somewhat rounded at the base. The basal margin is regularly semi- 

 ovate (that is, on the supposition that the ovoid be divided in the direction 

 of its greatest length) the most prominent part being about or behind the 

 middle, the upward trend being greatest posteriorly. The posterior end 

 is narrowly rounded, and judging by the lines of growth, a little angular 

 at is junction with the hinge border above. In front and below, the valves 

 seem to have been nearly closed ; behind they ga])e very slightly. 



The surface is marked by concentric ribs or rib-like folds, Avhich are 

 separated bj' narrow grooves. Both are ver}- irregular in their disposi- 

 tion, and are often partly divided longitudinall}^, so that they are rarely 

 continuous from efnd to end. There are also a few, very faint, radiating 



