1 
PACHYMYA*. 
Gen. Char, Shell bivalved, transversely elongated, 
very thick, sub-bilobate, with the beaks near 
the anterior f extremity. Ligament partly im- 
mersed, attached to prominent processes or 
fulcra. 
A strong analogy exists between this genus and Mo« 
diola, evinced by the position of the beaks, the elongated 
form of the valves, and the partial separation of the an- 
terior portion into an imperfect lobe. But a closer in- 
spection shows that it is rather related to Cypricardiaand 
several other genera which have a comparatively short 
ligament, and that, fixed upon a strong prominent part of 
the shell within the edge, not linear and affixed to nar*- 
row edges, as in most of those thin shells that are allied 
to Mytilus. The great thickness of the valves, their 
depth, and the ridge that crosses them obliquely, help to 
distinguish this from other genera, independently of the 
hinge teeth, which we regret not having seen. 
It is probable that several fossils hitherto described 
as Modiolae will, when better known, prove to belong 
to this genus. 
* From 7 rot, i xps ( crassus ) and Mya, another genus which this 
resembles. 
f We purpose in this volume to use the terms anterior and 
posterior in their correct sense, as pointed out by the situation of 
the mouth of the animal. 
Vol. VI. 
