CLINTON GROUP. 
87 
479. 1. POSIDONIA1 ALATA. 
Pl. XXVII. Fig. 4. 
Posidonia 1 alata. Hall, Geol. Rep. 4th Dist. N. York, pa g. 72, fig. 7. 
Shell suborbicular, compressed, inequilateral, alate posteriorly, rounded before and on the 
base ; hinge-line direct; beak slightly elevated above the cardinal line ; surface marked with 
concentric undulations and finer striae. 
This shell can not with propriety be referred to Posidonia, and it seems equally inappro¬ 
priate to refer it to any other established genus. The shell is distinctly alate posteriorly, though 
there is no sinus at the junction of the wing with the body of the shell, as in Avicula. The 
surface is smoother than in most species of Posidonia, though the external form is not suffi¬ 
cient to distinguish it. 
The specimen figured is the only one found in this group, but a similar or identical species 
occurs in the Niagara group. 
Position and locality. In the upper green shale of the Clinton group at Rochester. 
Genus PYRENOMCEUS {rum. gen.). 
[Gr. 'ffuprjvos, nucula, and ofioiwj, similis .] 
Shell equivalved, inequilateral; umbones prominent, beak elevated ; a strong muscular im¬ 
pression near the anterior extremity ; posterior muscular impression unknown ; characters of 
the hinge not fully ascertained ; surface concentrically striated. 
This genus is constituted to receive those shells having the general form of Nucula, but 
which are destitute of the teeth or crenulations so characteristic of that genus. The shells in¬ 
cluded under this genus have not the transverse fold or clavicle which marks the Genus 
Cleidophorus ; and although resembling in some degree the Modiolopsis in the anterior mus¬ 
cular impressions, they are contracted towards the posterior extremity. 
Many shells are referred to the Genus Nucula, simply from external form, and which are 
entirely destitute of teeth or crenulations on the hinge-line. The present genus, with Clei¬ 
dophorus, will include the greater proportion of those at present known to me. 
480. 1. PYRENOMCEUS CUNEATUS («. sp.). 
Pl. XXVII. Fig. 3, and 12 a, b, c. 
Shell robust, sub-cuneate ; beak strongly elevated, and the umbo very convex or even gib¬ 
bous ; anterior extremity very abruptly rounded ; posterior side elevated, rapidly narrowing 
from the beak to an acute extremity ; surface marked by equal concentric striae, and in older 
shells some stronger folds ; cast marked by a prominent anterior muscular impression. 
