218 MOLLUSCA FROM THE CRAG. 
regularity of growth. The principal difference between this and @. fragilis is a larger 
posterior side, which is biangulated, the posterior dorsal portion not sloping off to a 
point as in the recent shell: the laminze in our fossil are also larger, more remote, and 
the palleal sinus extends only to a line drawn perpendicularly from the umbo; in 
G. fragilis it projects beyond. The right valve of our shell has two large diverging teeth, 
while the left one possesses one large central, triangular, bifid tooth, with a cavity on 
each side, and two small rudimentary teeth; a large fulcrum for the ligament ; the 
laminze are prominent, and erect, not reflexed. 
Tellina fragilis, Linn., is found fossil in the Sicilian Beds, as, also, quoted in the 
Faluns of Touraine ; but I have not seen it from the Crag. 
Donax,* Linnaeus. 1758. 
Tettina. Adans, 1757. Latona.—Schum. 1817. 
Cuion. Scopoli, 1777. Hecusa. Id. 
Cuneus. Da Costa, 1778. Ientcenta? Id. 
Srrruta. Chemn. 1782. Carsa. Lam. 1818. 
CAPIsTERIA. Gevers, 1787. Sec. Gray. Donacina. Ferus. 1821. 
PERON#HA, PERONHODERMA (sp.) Poli, 1791. Eeeria? (sp.) Lea. 1833. 
Generic Character. Shell transverse, inequilateral, equivalved, more or less wedge- 
shaped ; posterior side the shorter; surface generally smooth and glossy, sometimes 
finely striated or decussated ; covered by an epidermis in the recent state. Margin 
plain or crenulated. Hinge composed of two cardinal teeth in one valve, and one in 
the other, with more or less developed lateral teeth. Muscular impressions ovate with 
a large and deeply indented mantle-mark. Ligament external. 
Animal somewhat oblong, the mantle open in front, with fringed or partially fringed 
margins. Siphons not very long, separated their entire length; foot large, sharp 
edged, and pointed. 
The principal distinction between this genus and that of Ze//ina is the truncation 
of the posterior side, and the general wedge-shaped form of the shell, as also the 
absence of the flexous fold in the margin, and it is in general more inequilateral, but 
the two genera are very closely allied. Ze//ina is made the type of a family by the 
malacologists, and the present genus is alike honoured with a similar distinction, upon 
what grounds they are so widely separated I am unable to discover. 
A fossil from the Carboniferous Period is described under this name, but its true 
generic position is doubtful, and there is no certainty of its existence in the fossil state 
anterior to the older Tertiaries. 
* Etym. dora, a sea-fish. Pliny. 
