102 PAUEONTOLOGY OF NBW-TORK. 



141. 4. LEPT^NA ALTERNATA.* 



Pl. XXXI. Figf. 1 a - n ; «nd Pi> XXXI. A. Figs. la-h. 



Leptmna alltmaia. CoiniAD, Ann. Geol. Rep. New- York, 183S, p. 115. 

 Strophomena allemala. Id. lb. 1839, p. 63, table of organic remains. 



— — In. lb. 1840, p. 201 : regarded in this place as identical with Orthis alternata of 



SowrRBY. 



— — Id. lb. 1841, p. 37. 



— — Emmons, Geol. Report, 1842, pag. 375, fig. 3. 



Ltptana alternata. VERNErn., Pal. of Russia and the Ural Mountains, 1845, pag. 225, pi. 14, fig. 6 a, b. 



— delloidea. Id. lb. pag. 222, pi. 14, tig. 5 a, b. 

 Compare Orthis alternata, Sowbrbt, Sil. Researches, pag. 638, pi. 19, fig. 6. 

 Also Strophomena natuta, Conrad, Jour. Acad. Nat. Sciences, Vol. viii. p. 20O. 

 — — Emmons, Geol. Report, pag. 403, flg. 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 reflected 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 striae, 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, iq 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 difiicult 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 diffierences 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 Leptana alternata, in 1S38 ; and the name was 

 known and continued in use in New-York till the publication of the final Reports ( the generic term Strophomena 

 being subftituted for leptana). When Mr. Murchison's Silurian Retearchet was received in this country, we 

 found that Mr. Sowerby had proposed the name of Orthii alternata for a lower Silurian species, which is doubtless 

 a true Lept.kna ; and not knowing certainly that it was an identical species, Mr. Conrad subsequently proposed 

 for our species the name of trentoneruis in his manuscript notes. M. de Verneuu. has adopted the name Lepttena 

 alternata in his PaUeontology of Ru$ria 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 Sowerbt, the specific term trentoneruis may 

 be adopted. The present name has precedence over all others applied in this country to the same species. 



