In speaking of the Crenatula, we gave the characteristic distinction,
which regarded it natural and expedient to make the Perna form a
different genus.
Shell irregular, very compressed, foliaceous, sub-equivalve, rather
variable form, gaping at the anterior part of the lower extremity;
summit very small, hinge straight, vertical, without teeth; ligament
multiple, and inserted in a series of longitudinal parallel furrows.
Perna ephippium.
P. obliqua.
P. isognomon.
P. avicularis.
P. femoralis.
P. canina.
P. marsupiom.
P. sulcata.
P. vulsella.
P. nucleus.
_P. femoralis._ The femoral Perna. Pl. 14, fig. 1.
Species elongated, and with appendages like ears.
_P. vulsella._ The tweezer Perna.
Species elongated, without earlike appendages, or having very small
ones.
_P. ephippium._ The saddle Perna.
Species round, compressed, very pearly in the interior; very slightly,
if at all, auricled; margin acute; exterior purplish brown.