102 
PALAEONTOLOGY OF NEW-YORK. 
141. 4. LEPT/ENA ALTERNATA.* 
Pl. XXXI. Figs. 1 a - n; and Pl. XXXI. A. Figs. 1 a - h. 
Leptcena alternata. Conrad, Ann. Geol. Rep. New-York, 183S, p. 115. 
Strophomena alternata. Id. Ib. 1S39, p. 03, table of organic remains. 
— •— Id. Ib. 1840, p. 201 : regarded in this place as identical with Orihis alternata of 
Sowerby. 
— — Id. Ib. 1841, p. 37. 
— — Emmons, Geol. Report, 1842, pag. 375, fig. 3. 
Leptcena alternata. Verneuil, Pal. of Russia and the Ural Mountains, 1845, pag. 225, pl. 14, fig. 0 a, b 
— deltoidea. Id. Ib. pag. 222, pl. 14, fig. 5 a, l. 
Compare Orthis alternata, Sowerby, Sil. Researches, pag. G3S, pl. 19, fig. 6. 
Also Strophomena nasuta, Conrad, Jour. Acad. Nat. Sciences, Vol. viii. p. 260. 
— — Emmons, Geol. Report, pag, 403, fig. 3. 
Broadly semioval; length and breadth about as 12 to 15 ; hinge line, in perfect speci¬ 
mens, a little longer than the width of the shell, slightly rellected at the extremities, which 
sometimes become short acute ears ; cardinal area narrow, the callosity of the ventral valve 
nearly filling the triangular foramen of the dorsal valve; beak uniformly perforated with 
a minute circular opening ; dorsal valve depressed convex, sometimes more convex in the 
middle, suddenly deflected near the margin and flattened towards the cardinal line ; ventral 
valve concave, gradually or sometimes suddenly inflected towards the basal margin ; 
surface marked by fine rounded radiating strife, which alternate at unequal intervals with 
coarser ones ; striae increasing in number towards the margin of the shell, crossed by fine 
elevated concentric lines and a few imbricating lines of growth. 
In this species the striae are usually of two sizes, the coarser and more elevated ones 
having from four to six finer ones between ; the latter increase in number as they recede 
from the apex, and one of them, in the centre of the fascicle, becomes enlarged, and rises 
above the others. Some of the specimens figured are strongly marked individuals, where 
these characters are very distinctly preserved ; but there are many variations from the type 
of the species, and some others where it is difficult to decide their true relations when we 
have not a series showing their gradations. 
The characteristic developments of the shell depend on the circumstances of its existence 
and the nature of the sediment. There are other differences which are due to causes beyond 
our knowledge; for, in the same locality, and where great numbers of them must have 
lived under precisely similar circumstances, we find considerable variation in character. In 
* This species was proposed by Mr. Conrad, under the name of Leptcena alternata, in 1838; and the name was 
known and continued in use in New-York till the publication of the final Reports (the generic term Strophomena 
being substituted for Leptcena). When Mr. Murchison’s Silurian Researches was received in this country, we 
found that Mr. Sowerby had proposed the name of Orthis alternata for a lower silurian species, which is doubtless 
a true Leptjena ; and not knowing certainly that it was an identical species, Mr. Conrad subsequently proposed 
for our species the name of trentonensis in his manuscript notes. M.de Verneuil has adopted the name Leptcena 
alternata in his Palceontology of Russia and the Ural Mountains, and I have continued the same in the present 
work. If, hereafter, it should be found to conflict with the species of Sowerby, the specific term trentonensis may 
be adopted. The present name has precedence over all others applied in this country to the same species. 
