948 : MR. T. H. WITHERS ON 
Measurements.—The complete shell is very minute, its dimen- 
sions being :-— 
Length (from base of rostrum to apex of carina) 2:7 mm. 
“a (from apex 2 - ) 2-1 mm. 
Greatest breadth’ .eceeer 2-2-2: ose ee eee ear 1-7 mm. 
: height (from apex of tergum to base)... 1-6 mm. 
The isolated valves are somewhat larger, and indicate that the 
complete specimen is a young individual. Their dimensions 
are: 
Movable scutum: length (from apex to middle of basal 
margin} 2‘8 mm.; breadth (greatest) 1*2 mm. 
Fixed scutum: length (from apex to middle of basal margin) 
2-7 mm., when complete, ca. 2°9 mm ; length (from apex to rostral 
angle) 3:1 mm. ; breadth (greatest) 2°7 mm. 
Fixed tergum: length 2-4 mm.; breadth 1:7 mm. 
Description of Specimens. 
Although the complete specimen is of great importance as 
showing the relative position of the valves, little can be seen of 
their inner structure. Moreover, the valves are covered exteriorly 
by a film of chalk, or may be a secondary deposit of silica, which 
serves both to obscure and unduly to emphasize the bead-like 
ornament. This is no doubt due to the shell having been pre- 
served in a hollow flint, for preservation in flints is apt, especially 
in highly ornamented forms, to make the ornament more 
pronounced. Owing to its importance it is too dangerous to 
attempt to clean this minute and delicate fossil, and it will have 
to be studied in its present condition. 
The isolated valves vary considerably both in ornament and 
structure, but while there is no doubt as to the identity of the 
fixed and movable scuta and terga, it is not at all certain that 
the rostral- and the carinal-latus belong to this species. In the 
complete specimen the rostral-latus is much broken, and the 
outer structure of this and the carinal-latus somewhat obscured. 
Without being able to disarticulate these valves it is impossible 
to compare them with the supposed isolated examples, of which 
-even the two rostral latera differ from each other. It is pro- 
bable that the variation in ornament is due to varying states of 
preservation. 
Vhe shell is broadly oval in outline, much elevated, the walls 
perpendicular on each side, and the movable scutum and terguim, 
when in position, would be inclined at an angle of about 30° with 
the base; rostrum and carina at either end, the rostrum wider 
than the carina and slightly more developed on the right side, the 
space between the rostrum and carina occupied on one side by 
the fixed scutum and tergum, and on the other by the carinal- 
latus, the rostral-latus for its whole length overlapping the lateral 
portion of the rostrum. 
Valves highly ornamented, the ornament consisting of strong 
