1889 .] 
309 
[Foerste. 
outline of the muscular scars in the ventral valve being approxi- 
mate and rounded, or at least not spreading ; the ventral valve is 
convex, the muscular scars of the dorsal valve form a semicircular 
outline of which the concave side lies towards the beak. 
Orthis flabellulum, Sowerby, has muscular scars in the ventral 
valve with their anterior outline spreading, forming a concave 
curve, the entire form being subquadrangular, instead of circular 
or subtriangular as in the last, the ventral valve is frequently flat- 
tened anteriorly, the anterior margin of the muscular scars of the 
dorsal valve being more distinctly defined and approaching closer 
to the beak. The postero-lateral angles of the shell are as a rule 
more rounded. 
Orthis flabellulum, Pal. New York, Yol. n, PI. 52, Fig. 6, com- 
bines features of both of these forms. The ventral valve is decidedly 
flattened, especially anteriorly, but still slightly convex, and never 
concave anteriorly as in Orthis flabellulum, Sowerby. The muscular 
scars of the dorsal valve anteriorly are quite prominent, but do not 
approach one another closely ; they take a position intermediate 
between those of the two species above cited. The muscular scars 
of the ventral valve correspond exactly with those of Orthis calli- 
gramma. In the angularity of the postero-lateral margins the 
New York specimens exceed the typical forms of Orthis calligram- 
ma, but may be compared with the otherwise very different variety 
proava, Salter. It undoubtedly belongs to Orthis calligramma , 
Dalman. 
Specimens illustrated by fig. 7 of the same plate, cannot be dis- 
tinguished from those of fig. 6, as a rule, except by the slightly 
greater convexity of the ventral valve, if our determinations are 
correct. 
Orthis fasciata , Hall, illustrated by fig. 8 on the same plate, I know 
only from a single cast of the dorsal valve. In it the cardinal line 
is but slightly extended beyond the general width of the shell and 
does not form acute wings. The muscular impressions suggest 
Orthis calligramma as just identified, but the elevation separating 
the same is more prominent and extends farther towards the anterior 
margin. The valve is somewhat flattened anteriorly. As regards 
the fasciation of the radiating striae, they present no features not 
found in finely striated forms of Orthis calligramma from Alabama, 
later to be described, and the produced cardinal line is also occa- 
sionally found in these. While I am therefore not at all prepared 
