CLINTON OKOUP. 



87 



479. 1. POSIDONIA'? ALATA. 



PL. XXVII. Fig. 4, 

 PoBidonia 1 alata. Hall, Geol. Rep. 4th Dist. N. York, pag. 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 stria2. 



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 P YRENOMCEUS (no-y. ^en.). 

 [Gr. irupiio?, nucula, and ofioiug, similis.] 



Shell equivalved, inequilatural ; umbones prominent, beak elevated ; a strong muscular im- 

 presssion near the anterior extremity ; posterior musculer 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 destittte 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 CUNEATTS (n. 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. 



