MELANIANA. 63 
MARTE’SIA. Leach. A genus composed of those species of 
Pholas, Auct. which areclosed at both extremities. Hx. P.clavata. 
MEA’SUREMENT. The most approved method of stating the 
measurements the various kinds of shells is as follows: Of sym- 
metrical, convolute univalves, the length is from anterior to poste- 
rior; the depth from ventral to dorsal; the breadth, from side 
to side of the aperture. Of symmetrical conical umwalves, length, 
from front to back; breadth, from side to side; depth, from 
apex tobase. Of spiral wnivalves, length, from apex to anterior 
of the columella or axis of the shell; breadth, across from the 
outer lip to the opposite side. Of non-symmetrical bivalves, the 
length is from the anterior to posterior margin; breadth, from 
the greatest convexity of one valve to the corresponding part 
of the other; depth, from the dorsal to the ventral margin. 
MEGADES’MA. Bowd. (Meyas, megas, great; Sesya, desma, 
ligament.) Poramopuiia, Sow. GaraTHEa. Lam. 
ME’GALODON. Sow. (Meyas, megas, great; odes, odos, tooth.) Fam. 
Cardiacea, Lam. Descr. Equivalve, longitudinal, acuminated 
at the umbones, thick; hinge forming an incrassated septum 
across the cavity of the shell, with a large bifid tooth in the right 
valve and one irregular and one pointed tooth in the left; liga- 
ment long, external. Obs. This fossil resembles Mytilus in 
general form, but differs in the thickness of the shell, and in the 
incrassated hinge, with the large tooth. Fig. 127, M. cucullatus. 
MEGARIMA. Rafinesque. A genus proposed to include species 
of Terebratula, Auct. which are smooth and nearly equivalve. 
T. levis, T. crassa, T. truncula. 
MEGASPIRA. Lea. (Meyas, megas, great, spire.) M. Ruschen- 
bergiana, fig. 294, is a pupiform land shell, remarkable for the 
length ofits spire, which consists of no less than twenty-five 
close-set, narrow, gradually increasing whorls. 
MEGATRE’MA. Leach. A genus composed of those species of 
Pyreoma, Auct. which have a large aperture. Fig. 33. 
MELAM’PUS. Montf. Conovulus, Lam. A genus composed of 
species of Auricula, Auct. of a conical form. A. conoidalis, 
fig. 298. 
MELA‘NIA. (Mehas, melas, black.) Fam. Melaniens, Lam. Ellip- 
sostomata, Bl. Descr. Turrited; spire generally elongated, 
acute; aperture entire, oval or oblong, pointed at the posterior 
extremity, rounded anteriorly, with a kind of indistinct canal 
or sinuosity; epidermis thick, generally black. Obs. In common 
with other fresh-water shells, the Melani are frequently found 
with corroded apices. This genus is known from Melanopsis by 
the absence of the notch at the anterior part of the aperture. 
The Melaniz occur in rivers of warm climates. The fossil 
species are frequent in the upper marine formations. Fig. 313, 
M. subulata. 
MELANIA'NA. Lam. A family belonging to the first section of 
