224 MOLLUSCA FROM THE CRAG. 
Length, \ inch. 
Locality. Cor. Crag, Sutton. Recent, Channel Islands, Hebrides, and Bergen. 
This delicate and elegant shell is rare in my cabinet; about half a dozen small 
specimens and a few fragments are all that I have seen. It much resembles the 
young of Ps. vespertina, but is rather more rounded on the posterior side, and it is a 
deeper or more tumid shell. The hinge teeth are small, though apparently a trifle 
larger than those of the recent shell. The palleal smus is large and deep, extending 
inwards to a little beyond the hinge denticles. It is said to extend in the living state 
from the Channel Islands to the coast of Bergen, and ranges vertically from five to fifty 
fathoms. 
TELLINA,* Linneus, 1767. 
PrrasuncuLus. Rumph. 1705. Petiver, 1713. Arcopaaia. Leach MS. 1816. Brown, 1827. 
CHAMELEA (sp.) Allein. 1753. Puyitopa. Schum. 1817. 
PERON@A ET PERONEODERMA (sp.) Poli. 1791. TeLiinipes. Lamk, 1818. 
TELLINARIUS (sp.) Dum. 1806. Macoma. Leach, 1819. 
AncuLus. Megerle, 1811. Lomicota. Jd. 1819. Fide Gray. 
Omata. Schum. 1817. Homata. (sp.) Agassiz. 
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Generic Character. Shell transversely ovate, or suborbicular, generally inequilateral, 
plain or smooth, but more frequently sculptured or ornamented ; posterior side more 
or less angulated, with an irregular flexuosity produced by a slightly sinuated form in 
the posterior part of the ventral margin. Hinge with generally two cardinal and two 
lateral teeth in each valve, the latter, in some species, are obsolete. Palleal impression 
deeply sinuated. Ligament external. 
Animal of the form of the shell, having the mantle open in front, with the margins 
fringed ; siphons long, sometimes four or five times the length of the shell, separate 
throughout, the orifices plain or indistinctly toothed; foot large, triangular, and 
compressed. 
The character by which this genus is said to be most distinguished is the flexuous 
fold, or slightly twisted form of the posterior side of the shell; this, however, is not 
always discernible, thereby merging into Sanguinolaria and Psammobia which are 
characterised by the absence of the fold, and the want of lateral teeth, though in some 
species of this genus the latter character is by no means permanent. The outward 
form of Zellina is exceedingly variable, some species being nearly orbicular, while 
others are much elongated; in general the shells are slightly compressed, somewhat 
unequal in size, one valve being more tumid than the other, especially on the posterior 
side, where the ligament is placed; and this side is generally, though not always, the 
shorter of the two. 
* Etym. TeAdivn, the name of a mussel. 
