438 PALEONTOLOGY OF NEW YORK. 



Valves regularly convex below, becoming gibbous in the middle and above. 



Beaks anterior to the middle of the length, attenuate, slightly directed 

 forward. 



Surface marked by from forty-five to sixty regular, flattened plications, 

 with narrower interspaces ; crossed by closely arranged, undulating, lamel- 

 lose concentric striae. 



Three specimens have lengths of 42, 45 and 55 mm. and are of nearly 

 equal heights respectively. 



This species differs entirely from any of the preceding forms, except L. orbi- 

 culare, from which it is distinguished by a longer truncation upon the posterior 

 side, and by more numerous and flattened plications. 



The typical specimen is of nearly equal length and height, and is marked by 

 about sixty flattened plications which are not quite properly represented in fig. 

 28 of plate Ixxi. The prevailing number of rays is from forty to fifty. 



The typical specimen of L. acutirostrum has the anterior and posterior mar- 

 gins abruptly infolded, giving the shell a much narrower aspect than when in 

 its natural condition. The plications are more slender than in the prevail- 

 ing forms referred to L. ornatum, and from the infolding of the margin those 

 of the anterior end are invisible in the figure. The specimen figure 31, subse- 

 quently referred to the same species, has a greater proportional height than 

 the prevailing forms of L. ornatum, the plications are also narrower and with 

 narrower interspaces. A comparison of these forms with larger collections, 

 showing numerous intermediate phases, makes it impossible to draw lines of 

 specific distinction, and it is dou})tful if any fixed varietal form exists. 



Formations and localities. In the shales of the Portage group at Cashaqua 

 creek, Livingston county, and in the arenaceous shales of the Chemung group, 

 at Elmira, N. Y. 



