102 
THE GEOLOGIST. 
Genus Stropiiomena. Eafmesque. 1820. 
This palseozoic genus appears to have been as much restricted in its vertical 
range as was Orthis, for but a single species is known to me from tlie Scottish 
Cai-boniferous strata, and no well authenticated example of the genus a])pears 
to have been observed in any of the subsequent periods. The shells of which 
this group is composed vary considerably in shape and character, beuig 
generally semi-circular, witli a long straight hinge-line, the ventral valve being 
eitlier convex or concave, while the dorsal one usually follows the cui-ves of the 
other, 
XXIV. — Steophomena riiomboidalis, Wahl., var. analoga, PhiUips. PI. i., 
figs. 26-33. 
Anomites rhomhoidalis, Wallenberg, Acta. Soc. Ups., vol. iii., p. 65, No. 7, 1821. 
= Prodiida depressa, Sowerby, Min. Con., tab. 459, 1823 ; Producta anal- 
oga, Phillips' Geol. of York, vol. ii., p. 215, pl. vii., fig. 10, 1836. etc. 
Of this species there are two well marked varieties, the first or typical 
one varies somewhat in shape, but is more often semicircular, with a very 
long hinge-hne. The ventral valve is geniculated, or, in other words but slightly 
convex or flattened up to a certain distance, and up to a certain age, when 
the valve becomes suddenly deflected downwards at ahnost right angles. 
The margin is undulated ; concave near the cardinal angles, it afterwards bmges 
out to form in front another slight inward or outward curve. On the flattened 
portion of the valve there exists a variable number of slightly undulating and 
irregular concentric wrinkles which turn outwardly towards the cardinal angles, 
and thus follow the marginal curves. The enthe surface is also covered with 
numerous thread-like radiating striae, and a small circular foramen is generally 
observable in the young, or up to a certain age, when it becomes obliterated 
or cicatriced in the adult. The dorsal valve usually follows the curves of the 
opposite one, and is similarly wrinkled and striated. The area in both is 
narrow and sub-marginal, with a small fissure in the ventral one, partially covered 
by a pseudo-deltidium. In the interior of the ventral valve two diverging 
teeth articulate with corresponding sockets in the opposite valve. The muscu- 
lar impressions in this valve are margined by a semicircular ridge, continued 
from the base of the teeth, and curving on either side so as to produce a saucer- 
shaped depression ; the occlusor leaves a scar on either side, close to a small 
median ridge, the devaricator filling on either side the anterior portion of the 
cavity ; the ventral, adjuster, and pedicle muscles do not appear to have pro- 
duced any very definite scars, but it is highly probable that an attachment for 
these muscles existed in the posterior portion of the saucer-shaped depression 
above described, from the fact that a small circular peduncular foramen is also 
sometimes observable at a short distance from the extremity of the beak, and 
which denotes that a pedicle muscle must have existed, although the foramen 
became closed as soon as the animal found it could dispense with the moorings 
required dui'ing the early stages of its development. In the interior of the 
dorsal valve the cardinal process is divided into two lobes, and not coimate with 
the diverging socket-ridges. Prom the base of this a sliglit median ridge runs 
down ana separates the two pau-s of occlusor scars, which are bordered by pro- 
minent ridges. The vascular unpressions consist of large primary vessels, wliich 
run at once direct to a short distance from the frontal margin, when they be- 
come reflected on either side to surround the ovarian spaces. 
Tlie second variety, or Lept. distorta of J. de C. Sowerby (Min. Con., tab. 
615, fig. 3) is more properly speaking a malformation of the Slroph. riiom- 
boidalis, var. analoga, and in which the dorsal valve becomes convex instead of 
concave ; but aU the other characters are similar to those of the typical shape. 
