BIVALVIA. 93 
rounded projection extends from the umbo to the base of the siphonilateral margin ; the 
radiating striz are close and regular upon the pedal and ventral regions, but upon the 
dorsal slope of the siphonal region these rays are more than usually distant, and are some- 
what nodulous ; it is very inequilateral, with a recurved and rather prominent umbo. 
CUCULLAA.! Lamarck, 1801. 
Generic Character. Shell equivalve, inequilateral, trapeziform or subquadrate, ven- 
tricose ; valves closed and striated; umbones remote, separated by a wide and concave 
ligamental area; anal muscular impression bounded by an elevated ridge ; hinge linear, 
furnished with a few teeth, generally lateral and oblique, but parallel with the hinge-line at 
the extremities ; connexus ligamental. 
The shells of this genus approach so closely to some of the Arce, that it is doubtful, in 
the opinion of several naturalists, if there be any good character by which the two can 
be generically separated. The principal distinction is its subquadrate outline and inflated 
form, for many of the Ark shells of the older rocks have their dental apparatus with a very 
similar. arrangement, the lateral teeth being few and oblique, sometimes parallel with the 
hinge-line. Mr. Lycett proposed a genus under the name Jacrodon for certain fossils of 
the Oolitic Formation, in consequence of the hinge-denticles differing somewhat in their 
number and position; those on the pedal side of the margin being almost at right angles 
to the hinge-line, while at the opposite extremity they are parallel with it, appearing thus 
to combine or unite the two genera, 4rea and Cucullea. The British species, Arca rari- 
dentata, has the teeth much inclined on both sides. Very many fossils have been placed in 
this genus, beginning as low as the Silurian Rocks and ranging up to the present period ; 
only one living species is known which truly resembles the typical form, and that is an 
Oriental shell. Some of the fossil species have the umbones inflected in a subspiral 
manner, but a commencement of this form may be seen in some of the Arce. 
Cucutuma pecussata, Parkinson. Tab. XVII, fig. 8, a—e. 
Cucuttma pEcussaTa. Park. Org. Rem., vol. iii, p. 171, t. xiii, fig. 1, 1811. 
—_ — J. Sow. Min. Conch., t. 206, figs. 3, 4, 1818. 
_ CRASSATINA. Morris. Catal. Brit. Foss., p. 197, 1854. 
— _ Prestwich. Quart. Journ. Geol. Soe., 1854, p. 109. 
- 
Spec. Char. Testé transversd, ovato-oblongd, gibbosd, incrassatd, obliqud, inequilaterali, 
decussatim striatd, in medio compressiusculd ; pedi-regione brevi, obtusd, siphoni-regione 
1 Ety. Cucullus, a hood. Type, Arca cucullus, Linn. 
