1847.] SHARPE ON THE PALHOZOIC ROCKS OF N. AMERICA. 179 
1846, Mr. King proposes to limit the genus Leptena to the species 
which are wrinkled transversely, as L. depressa and rugosa, and to 
include in Strophomena the rest of the species now classed in Leptena, 
as L. euglypha, transversalis, &c. 'There is here hardly difference 
enough to warrant a division into two genera, for with the exception 
of the transverse wrinkling of the valves, there is no distinguishing 
character between them ; and the name of Strophomena also is applied 
to a group not intended by its author, who had a flat, regular, sub- 
equivalve shell in view, and not such produced, irregular forms as 
Leptena euglypha, &c.: so that we cannot adopt this classification. 
Confining the genus Orthis as here proposed to the species with 
an open triangular foramen, we have a well-marked character which 
distinguishes it from the other two genera; but we still require cha- 
racters to divide Strophomena from Leptena. These will be found in 
the regular form and striation of Strophomena, with the valves nearly 
flat or regularly curved, in contrast to the irregular curving and stria- 
tion of Leptena, with the vaives strongly and suddenly bent over 
and produced in front. But there are more important internal cha- 
racters connected with the hinge-processes and the muscles of attach- 
ment which distinguish them. In Leptena the dental lamelle of 
the dorsal valve unite with an elevated ridge, which curves round and 
encircles the base of the muscles; the space devoted to the attach- 
ment of the muscles is nearly circular, and is completely surrounded 
by this elevation ; the inclosed space is divided longitudinally by a 
broad plate or ridge, from which a minor pair of muscles take their 
rise: in the ventral valve the bases of the muscles are equally con- 
fined by two elevated ridges encircling them. In Strophomena the 
dental lamellee of the dorsal valve are produced in a straight line, 
marking the limits of the muscles towards the sides of the shell, but 
there is no ridge or enclosure round the fore part of the muscular 
impressions, and a straight plate starting from the apex separates the 
bases of the two muscles: thus there are three straight plates in the 
interior of the valve which converge at the apex. The arrangement 
of the plates on the ventral valve is nearly similar. 
These distinctive internal characters are well exhibited in Mr. 
Davidson’s beautiful illustrations of the Brachiopoda, pl. 12 & 13 
of Charlesworth’s ‘Geological Journal,’ vol. i., where the interior 
arrangement of Leptena is fully shown in the figures of L. depressa 
and euglypha, and that of Strophomena in those of Orthis pecten, 
which belongs to that genus. 
The differences of internal structure above described appear the 
consequences of the following peculiarities in their respective animals ; 
that in Strophomena the base of the great pair of muscles covers a 
much larger proportion of the surface of each valve than in Leptena, 
and thus their attachment is less distinctly marked. On the other 
hand, there is a greater thickening of the shell in Leptena during 
the growth of the animal; and as this deposit of shelly matter takes 
place less under the base of the muscles, in old shells their place is 
marked by a deep depression. 
‘In Strophomena the dorsal valve is usually slightly elevated near 
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