138 MOLLUSCA FROM THE CRAG. 
recent British shell it is one of our rarest species. In a comparison with Montague’s 
specimen, now in the British Museum, I was not able to detect the slightest 
difference. 
Our shell may be further described as very nearly orbicular, though in some 
specimens, from a slight prominence of the umbo, the diameter is rather greater in 
height: the hinge is furnished with one rather obtuse and angular tooth in the right 
valve, and two lateral teeth, the posterior one is the more distant ; in the left valve are 
two diverging cardinal teeth, with two lateral dentations corresponding to the teeth of 
the opposite valve, and the ligament is placed behind the cardinal teeth in an oblique 
fossette : the adductor muscle marks are slightly unequal : the anterior one is somewhat 
elongated, but it has not the band-like form of the true Zucina: that by the mantle 
is quite entire: the interior is often furnished with numerous radiating striz, and the 
margin in very perfect specimens is finely crenulated. The exterior is ornamented 
with diverging or divaricating striz, or rather ledges varying from 25 to 30, they are 
slightly undulatory and have the ledge or elevated part on the upper side or towards 
the umbo, and are crossed by the lines of growth: the divergence is from an imagi- 
nary line a little on the anterior side, at an angle generally of about 100°. 
This is the only species with these peculiar markings that I have seen: the shell 
from the Older Tertiaries, which is abundant in the Hordwell Cliff, has the ligament 
placed wholly externally upon a projecting fulcrum, and the species from Bordeaux, 
with the same specific name, differs in the like character, as also do the West Indian 
shells. 
There are, probably, several species possessing these diverging and curving 
radiations, all of which have been united under the name of divaricata, and a long 
extension of Geological Age, as well as a wide Geographical distribution, have been 
given in consequence. The common West India shell, and the Older Tertiary fossils, 
also belong to the true Luczna. 
Lucina,* Bruguiére, 1792. 
Venus (sp.). Linn. 
TELLINA (sp.). Mont., 1803. 
Cyracuma. Leach, MS., 1819, fide Gray. 
Myrrea. Turt., 1822. 
Puacorpes. Blainv., 1825. 
Ortyeta (sp.). Brown, 1827. 
Generic Character. Shell equivalve, generally equilateral, lenticular, compressed, 
occasionally tumid; surface more or less ornamented with concentric striz or 
elevated ridges, sometimes with radiating striz or coste. Hinge usually with two 
diverging cardinal teeth in each valve, and two lateral teeth, which in some species 
* Etym. A Proper Name. 
