BIVALVIA. 93 



rounded projection extends from the umbo to the base of the siphonilateral margin ; the 

 radiating striae 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. 



CUCULL^EA. 1 Lamarcl; 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 Avith the hinge-line at 

 the extremities ; connexus ligamental. 



The shells of this genus approach so closely to some of the Area, 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 Macrodon 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, Area and Cucullcea. The British species, Area 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 Area. 



CucuLLiEA decussata, Parkinson. Tab. XVII, fig. 8, a — c. 



Cucull/EA decussata. Park. Org. Rem., vol. iii, p. 1/1, 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. Soc, 1854, p. 109. 



Spec. Char. Testa transversa, ovato-oblongd,gibbosd, incrassatd,obliqud, inaquilaterali, 

 decussation striata, in medio compressiusculd ; pedi-regione brevi, obtusd, siphoni-regionc 



1 Ety. Cucullus, a hood. Type, Area cucullus, Linn. 



