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 indentations 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 Lucina : that by the mantle 

 is quite entire : the interior is often furnished with numerous radiating striae, and the 

 margin in very perfect specimens is finely crenulated. The exterior is ornamented 

 with diverging or divaricating striae, 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 Old er 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 Lucina. 



LUCINA,* Bruguiere, 1792. 



VENUS (sp.). Linn. 

 TELLINA (sp.). Mont., 1803. 

 CYRACH.EA. Leach, MS., 1819, fide Gray. 

 MYRTEA. Turt., 1822. 

 PHACOIDES. Blainv., 1825. 

 ORTYGIA (sp.). Brown, 1827. 



''Generic Character. Shell equivalve, generally equilateral, lenticular, compressed, 

 occasionally tumid ; surface more or less ornamented with concentric striae or 

 elevated ridges, sometimes with radiating striae or costae. Hinge usually with two 

 diverging cardinal teeth in each valve, and two lateral teeth, which in some species 



* Etytn. A Proper Name. 



