BIVALVIA. 67 



LUCINA BELLONA, If Orb. Tab. VI, figs. 18, 18 a. 



LUCINA LIRATA, var. TRANSVERSA, TfArchittc. Mem. Soc. Geol. France, v, t. 26, f. 3. 

 BELLOXA, d Orb. Prod. Paleont., 1, p. 309. 



Testa transversd, ovato-orbiculari, plano-convexd, antice rotundatd, postice subsinuatd, 

 umbonibus mediants acutis, margine cardinali subrecto, oblique declivi, lunuld pared exca- 

 vatd; superficie plicis concentricis magnis irregularibus, striis densissimis impressis. 



Shell transverse, ovate, rather flattened, anterior margin rounded, posterior margin 

 sinuated ; umbones mesial and pointed ; hinge margin lengthened, straight, and oblique ; 

 concentric folds rather irregular, elevated, and impressed with longitudinal, densely 

 arranged, and very fine striations. An obscure elevation passes obliquely from the 

 umbones to the inferior and posterior border. This shell presents considerable variety in 

 its form and markings ; young specimens are much more compressed, and then* borders 

 are very acute, the general outline is nearly orbicular, and the concentric plications are 

 very distinct and regular : the adult shell becomes either of a suborbicular and convex, or 

 of a transverse and more depressed form, and in both varieties the concentric elevations are 

 placed at unequal distances ; the suborbicular variety has a more excavated lunule, and the 

 umbones are more directed forwards or oblique. The shell figured by M. D'Archiac, belongs 

 to the transverse variety to which our Great Oolite specimens belong, but we are not without 

 examples of the other form. It is absent in the shelly beds of the Great Oolite, being found 

 only in mudstones, or a fine calcareous muddy sediment which has become limestone. It 

 occurs very abundantly in the middle division of the Inferior Oolite in Gloucestershire, 

 associated with Nerincea, and a numerous suite of other Mollusks, but is almost absent 

 when the organic fades consists of Terebratula . It reappears in the upper beds of the 

 Great Oolite, forming a numerous colony in a compact marly rock, about one hundred 

 feet above the Fuller's earth, where it is seldom that specimens much better than nuclei 

 can be disengaged. 



Dimensions. Transverse variety: lateral diameter, 31 lines ; height, 25 lines; diameter 

 through both valves, 13 lines. Suborbicular variety: lateral diameter, 26 lines; height, 

 24 lines; diameter through both valves, 13 lines. 



Localities. In Great Oolite, two miles east of Minchinhampton. In Inferior Oolite, 

 along the outer escarpment of the northern and middle Cotswold hills. Also near Stam- 

 ford, and in other localities in Lincolnshire. 



LUCINA BELLONA, var. Dcpressa. Tab. VI, fig. 15. 



Testa transversa, subeequilaterd, orbiculatd, compressd et leevigatd ; margine superiori 

 antice subhorizontali postice recto declivi, striis concentris irregularibus. 



Shell transverse, subequilateral, orbicular, compressed and smooth, superior margin 

 anteriorly produced, and nearly horizontal, posterior margin straight and sloping ; base 

 regularly elliptical. 



