Vol. 58. | PROF. GROOM ON POLYPHYMA LAPWORTIH. 85 
and near the angles; it is marked off from the rest of the abe by 
a gentle depression, and thus forms a marginal ridge (7).' Along 
the posterior margin, in the best- mreceived examples, the narrow 
rim is raised only slightly above the level of the adjacent part of the 
shell. The dorsal margin, too, is as a rule slightly raised above 
the general level to form a narrow ridge, and is thence sharply 
bevelled off towards the hinge-line. 
The centre of the valve is occupied by a well-defined, broad reni- 
form elevation, the concavity of iron is directed obliquely upward 
and backward. The ventral limb (6) of this is more prominent 
than the dorsal (c), and at or near its free extremity is generally 
raised up to form a prominent rounded tubercle. Prof. Rupert 
Jones has suggested to me that this limb represents the ‘ gigot-lobe’ 
of Beyrichia. It is on this supposition that the orientation adopted 
here is based. 
In the antero-dorsal part of the valve are situated two tubercles. 
Of these, one (d), placed in a line with the free ends of the 
lobes 4 and c, is nearly hemispherical; the second (c), situated 
in front, in the best-examples is spindle-shaped, the axis of the 
spindle being directed obliquely upward and forward. These two 
tubercles. though sometimes apparently isolated, are usually more or 
less completely connected with the lobe ¢ by a low forked ridge. 
Towards the posterior end of the dorsal margin is a third smaller 
tubercle (a), sometimes very small and hemispherical, but usually 
elongated in a direction more or less parallel to the dorsal margin 
of the valve, with the bevelled edge of which it is, as a rule, nearly 
continuous at one point. 
Starting from a point close to the antero-ventral margin is a 
well-marked lobe (f) which, diverging from the margin, runs 
forward and upward, and ends near the lobe e. In the smaller 
examples it frequently appears to be spindle-shaped, but in larger 
and better specimens it is subtriangular. From a second point, 
on the postero-ventral margin, a more slender and less elevated 
ridge (g) runs upward and backward, and at the same time 
diverges from the edge and becomes submarginal. I have been 
unable to make certain that this ridge represents a feature originally 
present, for it is often poorly developed, and seems to occur at 
variable distances from the margin, sometimes indeed quite close 
to the latter; but its repetition with identical features in several of 
the best specimens favours the view that it is not of secondary 
origin. Indications of a small triangular lobe (2) are sometimes 
seen at the anterior angle in good specimens. 
The lobes just described rise up from the adjacent areas with 
tolerable distinctness, and where close together are separated by 
well-defined channels with rounded floors. The lobe c, however, 
often subsides gently into a flattened area characteristic of the 
hinder part of the valve. The two valves of the shell appear to be 
perfectly similar, and are sometimes preserved in juxtaposition. 
The dorsal and ventral edges appear to he wholly in a plane, and 
» The letters aj in parentheses refer to the text-figure, p. 84. 
