518 BRITISH PALAZOZOIC FOSSILS, [LaMELLIBRANCHIATA, 
posterior end sinuating the margin (which they look upon as distinctive of the Russian species) in our Derby- 
shire examples, and it is slightly indicated in Sowerby’s figure, under his synonymous C. elongatum. The 
other character which these authors rely on for separating the Russian and English species, viz. the smoothness 
of the posterior end, I find to be also of no value; for one of our specimens shews that the unworn shell is smooth, 
and without external radiating ridges from one end to the other, the ridges making their appearance here and 
there in patches wherever the external shell is worn through; the ridges seeming sharp, narrow, and distant 
when much worn, but broad, flat, and close together, when slightly worn. 
Position and Locality—Rare in the carboniferous limestone of Derbyshire. 
3srd Ord. Macrorracnia (Swain.) 
Shell usually equivalve or nearly so, holding a vertical position when alive*, symmetrical ; animal sym- 
metrical, mantle open in front, closed behind and forming two projecting tubes, either separated or united, 
forming a deep anal sinus in the pallial impression ; two or three adductor muscles ; gills forming two double 
laminze along the sides of the body. 
This group includes all the shells which have a sinus in the pallial impression. The families are: Ist, 
Corbulidw ; 2nd, Veneride; 8rd, Tellinide; 4th, Saxicavide; 5th, Solecurtide ; 6th, Solenellide ; 7th, 
Anatinide ; 8th, Myide ; 9th, Pholadide ; 10th, Clavagellide. 
8th Family. MYIDA. 
Shell, regular, very inequilateral, equivalve, oblong or elliptical, gaping at both ends ; pallial scar strong, 
with a very deep sinus ; hinge and ligament variable ; animal, mantle nearly closed, leaving a small slit for 
the protrusion of the foot; siphons enclosed in one very large extensile tube; two muscular impressions in 
each valve. 
Like the Pholadide all those shells have their siphons enclosed in a large, fleshy, very extensile tube ; they 
live deeply buried in the sand or mud, rising or sinking in their holes by the siphons and the foot, but never 
changing their places during their lives. 
Genera :—lIst, Solen ; 2nd, Leguminaria ; 38rd, Panopewa ; 4th, Pholadomya; 5th, Glycimeris; 6th, Soleni- 
mya; 7th, Mya; 8th, Lutraria ; 9th, Mactra. 
Genus. SOLENIMYA (Lamk.) 
=“SoLtemya” (Lamk.) 
Gen. Char.—Elongate, oblong, equivalve, very inequilateral, posterior end much the shortest; dorsal and 
ventral margins subparallel; anterior and posterior ends rounded, gaping ; surface covered with a very thick, 
glossy, horny periostraca, extending in jagged portions beyond the ventral margins; beaks very minute, incon- 
spicuous, close to the small posterior end ; cartilage forming a thick triangular mass, partly external and partly 
internal, close behind the beaks, supported internally on an oblique ensiform narrow plate, one edge of which is 
attached to the interior of each valve, extending from the beak backwards and downwards about halfway towards 
the respiratory margin; long, anterior, cardinal margin simple, erect, without teeth; posterior adductor small, 
ovate, within the cartilage plate ; anterior impression large, comma-shaped, both superficial ; pallial scar scarcely 
defined, with a minute sinus before joining the adductor at the short end, Anzmal, having the mantle closed 
in the short posterior part, and terminating in a short, inextensile siphon, open in front for the exit of a very large 
foot, the end of which is truncated and fringed ; one deeply pectinate gill-plume on each side. 
The sinus in the pallial scar is so minute that most observers describe it as entire. The posterior side 
being so very much shorter than the anterior, and the gills instead of being lamellated, forming two pinnate 
* Except the Corbulide, which are inequivalve, and rest in the sand in an oblique position (d’Orb.) 
