65 
nearly flat. Area of the vertral valve about one line wide at the beak, that of the dorsal 
valve being about half as wide. Both areas are striated, and they form with ohe another an 
angle of 90° or a little more. There is no foramen, but sometimes a flat triangular space with 
anarrow median callosity. 
Surface with numerous fine, equal or sub-equal striz, which increase in number in pro- 
ceeding from the beaks to the margins, both by bifurcation and intercalation. Six to nine 
striz occupy the space of a line; and these are crossed by close-set concentric strie, from 
ten to twelve in the space of a line. Besides the close cancellation thus produced, there 
are often irregular concentric undulations of both valves, and sometimes the bases of minute 
spines can be detected. 
Average examples have a width of about an inch by a length of about three quarters of 
an inch ; but the width on the hinge-line varies from half an inch to two inches. 
As regards the internal characters, Prof. Hall states (Pal. N. Y. Vol. iv. p. 100) that 
“the interior of the ventral valve is marked by large flabelliform divaricator muscular im- 
pressions, extending more than half the length of the shell; narrow above, with sides nearly 
straight, curving below and deeply divided in front, each division showing four or five lobes. 
The occlusor impressions are two semi-oval elevated spots a little below the apex, the centre 
becoming a thickened ridge or process lying beneath the place of the foramen, with a cavity 
on each side for the insertion of the bifureate dorsal cardinal process) * * * The mus- 
cular impressions of the dorsal valve are distinctly but not strongly marked ; these are sepa- 
rated above by a median ridge which divides in the bifurcating cardinal process, and this is 
supported on each side by an oblique pustulose ridge which gradually merges into the sur- 
face of the shell.” 
Strophomena perplana (Conrad), belongs to a 
group of Strophomenoid shells separated by Prof. 
Hall from Strophomena proper, to form a new 
genus, which he terms Strophodonta. The chief 
peculiarity of the forms included by Hall in 
his genus Sfrophodonta is the absence of a foramen 
in the area of the ventral valve; but as the 
forms in question are in most respects other- 
wise undistinguishable from the typical species of 
Fig. 20. a = . 
Strophomena perplana (Conrad). a A medium-sized Strophomena, it would seem advisable not to regard . 
individual, of the natural size: @ A portion of the sur- 4); a nace re ; . 
face of the same, enlarged. Corniferous Limestone. this peculiarity as being of more than at any rate 
sub-generic value. S. perplana is an extremely characteristic species of the Devonian Rocks of 
North America, and is in general readily recognised by its flattened form, its fine equal striz, 
and the close concentric striation of the surface in well preserved specimens. 
Locality and Formation.—Common in the Corniferous Limestone of Port Colborne, and 
lot 6, Con. 1, Wainfleet. 
77. StTROPHOMENA DEMISSA (Conrad). 
Strophomena demissa (Conrad), Journ. Acad. Nat. Sciences, Philadelphia, Vol. VIII, p- 
258, pl. 14, fig. 14. 
Strophomena demissa (Hall), Tenth Report on the State Cabinet, p. 137. 
Strophodonta demissa (Hall), Report on the Geologival Survey of Iowa, p. 495, Pl. III, 
fig. 5. 
__ Strophomena demissa (Billings), Journ. Can. Institute, New Series, Vol. VI, p. 341, figs. 
116-118. 
Strophodonta demissa (Hall), Pal. N. Y., Vol. IV, pp. 81, 101, 114. 
Shell very variable both in form and in dimensions. The form is usually semi-elliptical, 
but may be sub-quadrate or sub-triangular. Usually the width of the shell exceeds the 
length, but sometimes both dimensions are equal, and occasionally the width is less than the 
length. ‘The hinge line is usually equal to or greater than the greatest width of the shell, 
and the cardinal angles are often extended or auriculate. Specimens vary from a third of 
an inch to an inch and a half in width. 
The ventral valve is convex, sometimes strongly so, and the convexity is usually uniform 
and regular from the umbo to the ventral margin. The dorsal valve is moderately concave. 
5 
