68 
valve. Area of the ventral valve from one to three lines in width, transversely striated for. 
from one-third to one half of the distance from the beak to the cardinal angles. Dorsal area 
narrow and linear. 
“Surface marked by angular, sub-equal, interrupted or rugose strie, which are often 
made to appear more uniform by the partial exfoliation of the shell. The strie are bifur- 
cated two or three times before reaching the margin, and they are sometimes increased by 
intercalation. As usually preserved, they rise at intervals into sharp ridges or elongated 
nodes ; and in well-preserved specimens these are produced into short spines. When the shell 
is exfoliated, the surface between the striz is marked by minute pores or tubular openings, 
which communicate with pustulose elevations on the interior surface of the shell. In the en- 
tire cast, these pores mark the surface with considerable regularity, varying in size, and uni- 
formly larger below the middle, where the valve is more abruptly reflected. 
‘‘ The muscular impressions of the ventral valve are very large and broad, and the mar- 
gins distinctly limited by a curving elevated border, It is divided in the middle by a more 
or less developed septum, which terminates in a prominent callosity beneath the place of the 
foramen : this callosity gives a bilobate character to the cast of the rostral cavity. The 
place of the occlusor muscles is strongly marked, and that of the divaricator muscles distinctly 
bilobed. The muscular imprints are about two-thirds as long as wide, and the width is often 
nearly equal to half the width of the shel!. The muscular impressions of the dorsal valve 
are narrow and elongate, and are separated by a rounded median ridge, which divides above 
and continues in two diverging processes, the full extent of which is not shown in any of our 
specimens,” (Hall, Pal. N. Y., Vol. IV., p.94). ‘ 
Professor Hall places S. ampla in his genus Strophodonta, and states that the place of a fora- 
men is taken by a smooth triangular space with a narrow callosity along its centre. According 
to Mr. Billings, however, there is a small linear closed foramen, one-third of an line in width. 
It seems very doubtful if there-is any difference of specific value between S. ampla and the 
more ancient Lower Helderberg species, S. punctulifera | Conrad), S. Headleyana (Hall), and. 
cavuwinbona (Hall). Mr. Billings points out that S. ampla and S. punctulifera are substantially 
identical, the Corniferous specimens, however, being on an average larger than those figured 
by Hall from the Lower Heldelberg. I have, however, collected examples of S. punctulifera 
the dimensions of which are fully equal to those of the Devonian form. It would probably be 
wise, therefore, to substitute S. punctulifera for S. ampla, retaining the names Headleyana 
and cavumbona in the meanwhile as designations of varietal or sub specific value, thus carrying 
out Professor Hall’s own view that a more extensive series vf specimens will show that all these 
forms belong to “ one species presenting variations of size, degree of convexity, width of area, 
and difference of surface-markings due to the influence of the sediment and other surrounding 
conditions which affect the development of animal life.’ (For descriptions and figures of S. 
Headleyana, S. cavumbona, and S. punctulifera, see Paleontology of New York, Vol. IIT, 
p. 185, Pl XX., Figs. 1—3 ; p. 187, Pl. XXI, Figs. 1—3, and p. 188, Plate XXI, Fig. 4, 
and Pl. XXIII, Figs. 4—7. 
Strophomena ampla may as a rule be recognized by its peculiar form and the characteris- 
tic ornamentation of the surface. The commonest appearance is when the shell is partially ex- 
foliated. and the puncta between the strize are visible. Spines cannot usually be detected, 
but Mr. Biilings has figured a specimen in which the whole surface is adorned with short 
spines. 
Locality and Formation.—Corniferous Limestone of Port Colborne. 
81. STROPHOMENA NACREA (Hall). 
Strophomena (Strophodonta) nacrea (Hall), Tenth Report on the State Cabinet, p. 144. 
Strophodonta lepida (Hall), Geol. Report, Iowa, Vol. L., Part ii., p. 493, Pl. IIL, figs. 3, 
a: Ost: . 
Strophomena lepida (Billings), Journ. Can. Institute, New Series, Vol. VI. p. 344. 
Strophodonta nacrea (Hall), Pal. N. Y., Vol. IV., p. 104, Pl. XVIIL., figs. 1, a, 6. 
Shell small, from four to nine lines wide, and from three to six lines long, semi-circular, 
or sub-quadrate, the hinge-line crenulated and as wide as, or wider than, the shell below. 
Front margin rounded, cardinal angles rounded or extended and angular. Ventral valve 
