87 
Athyris clara (Billings), Canadian Journal, New Series, Vol. V. p. 274, Figs. 29-32. 
Meristella nasuta (Hall), Pal. New York, Vol. 1V., Plate XLVIII., Figs. 1-25. 
‘Nearly smooth, ovate or sub-rhomboidal; greatest width, about the middle; a short 
linguiform projection in the middle of the front margin ; both valves convex ; ventral valve the 
larger, with its beak strongly incurved. Length from one to two inches; width equal to, a 
little less, or a little greater, than the length. The ordinary size is about one inch and a half in 
length. 
~ «The ventral valve is strongly and smoothly convex, the outline evenly arched from beak 
to front, more abruptly curved above than below, the umbo prominent, the beak rather small, 
neatly rounded at the sides, and closely inéurved. The linguiform projection in the middle 
of the front margin is often a simple extension of a portion of the shell, without a sinus, but 
occasionally there is either a short, shallow depression, or a narrow rounded mesial fold, which 
seldom, however, extends towards the beak more than three or four lines. The upper half of 
the dorsal valve is sometimes evenly convex, but in general an indistinct, more or less broadly 
rounded carination, can be traced from the umbo along the middle to the front,. where it 
becomes abruptly elevated into a short, prominent, rounded fold, which extends into the lingui- 
form projection. On each side of the median line, this valve is gently convex, and often exhi- 
bits a rather flat slope to the lateral margins. The beak is strongly incurved, and appears to 
be deeply buried in the cavity beneath the umbo of the ventral valve. 
“ [fa line be drawn across the shell at mid-length, and another at one-fourth the length 
from the front, the greatest width will be found to range between the two. Many of the speci- 
mens are obtusely angular at the sides, and in such the margins above and below the angles 
are somewhat straight, the upper two sides converging to the beak, and the lower two to the 
linguiform extension in front, giving to the shell a rhomboidal instead of an ovate outline. 
“ At first sight, the surface appears to be smooth, with a few concentric squamose lines 
of growth. Oncloser examination, numerous indistinct, radiating lines may be seen. Of these 
there are from two to four in the width of one line, and they sometimes appear to lie beneath 
the surface of the shell. In very perfect specimens, the surface exhibits fine concentric striz, 
from ten to fifteen in the width of one line, and these are most distinct towards the front 
margin. 
“« Beneath the beak of the detached ventral valves, there is a wide, triangular foramen, not 
visible when the valves are united. The inside of the beak is entirely hollowed out into a 
deep pit or channel, which opens directly into the cavity of the shell. The impressions of the 
divaricator muscles occupy part of a sub-triangular space, the upper angle of which is situated 
just where the excavation beneath the beak opens out into the visceral cavity. The lower 
side of this space is nearly straight, and the two lower angles rounded. The lateral margins 
of the space are usually sub-parallel in the lower half, while in the upper half they approach 
each other, and meet above to form the upper angle. In some specimens the space is more 
nearly triangular, and it would appear, therefore, that its furm is a little variable. The length 
of the space is about one-third the whole length of the ventral valve, and its width at the lower 
margin a little less than its length. The lower margin is situated a little above a line drawn 
across the shell at mid-length. The lower three-fourths of the space is striated longitudinally, 
and divided into two equal portions by an obscure median groove. 
‘On each side, at the base of the foramen, there is a short, stout tooth. The dental 
plates below the teeth extend but a short distance into the visceral cavity, when, becoming 
suddenly much diminished in height, they form a low ridge along the upper margin of the 
muscular space. The upper part of the muscular space is deeply excavated into the substance 
of the shell, which is very thick and solid in the rostral half”’ (Billings). 
There is not much to add to the above quoted full description of this common Cornifer- 
ous species by Mr. Billings ; but there are a few points which may be noted. The nasute or 
linguiform extension of the front of the ventral valve, and the corresponding fold on the dor- 
sal valve, are only fully developed in adult examples. Hence in young examples, the front of 
the shell may exhibit nothing more than a hardly perceptible undulation. Small specimens 
are also proportionately more circular. The beak of the ventral valve is imperforate, appar- 
ently at all stages of growth ; but there is a triangular fissure beneath the beak, which by the 
incurvation of the beak is entirely closed in the perfect shell, and which can only be seen in 
the detached valve. 
Casts of the interior of the ventral valve of this species are extremely common in the 
