FAUNA OF THE TRENTON GROUP. 
15 
is of the same size and shape as the common American form usually 
referred to this species, but the striations are not the same, and agree 
with Dalman’s description. 
To Sardeson is due the suggestion of studying the striae of Dal- 
manella according to the number and arrangement of the fascicules, 
and it is when the shells are viewed in this way that the difference 
between the American and 'Swedish forms 1 is best seen. Sardeson 
pointed out that in Dalmanella rogata eleven principal striae extend 
to the beak or cardinal margin and that the shell is covered by lines 
produced by the bifurcation of these. The same sort of thing is seen 
in the Swedish form, hut as the branching takes place closer to the 
beak, there are more of the strong prominent strife. There are, as Dal- 
man says, two kinds of striations, about thirty elevated ones which 
reach the beak, and between them, shorter and finer ones, two or 
three of which lie between each pair of elevated ones. On Dalmanella 
r ogata this alternation of elevated and weak striae is not so readily 
observed, because many of the short striations are as coarse as the long 
ones. 
On the particular specimens at hand, there are the same number 
of striae on the Swedish and American specimens. Thus, on specimens 
of the same length (11*5 mm.) there are six in 3 mm. on the front of 
the pedicle valve of each. 
The difference between the two species is, therefore, small, con- 
sisting principally in the greater uniformity in size in the striations 
in the American species. The American species is also a little less 
circular than the Swedish form, the hinge being wider, and con- 
sequently, the cardinal area larger. The measurements compare 
as follows. 
D. testudinaria — length ll-<5 mm., width, 1 2 mm., cardinal area 
6-8 mm. wide, 
D. rogata — length 11*5 mm., width 12 mm., cardinal area 
8 mm. wide. 
Dalman's specimen was larger, 16 mm. long and 15 mm. wide. 
Dalmanella rogata. Shell sub-circular in outline, plano-convex; 
the pedicle valve moderately convex, with a well-defined median 
ridge; brachial valve slightly convex with a well-defined median 
sinus, sharp at the beak, but gradually becoming broader and in old 
specimens flattening out at the front. Surface marked by from 80 
to 120 fine striae, the result of the repeated bifurcations of about 
eleven primary ones which reach the apex. On the pedicle valve is 
a median striation which is stronger than the others. 
Horizon and Locality. Tery common throughout the Black River 
and Trenton of Canada and' the United States. 
