112 



PALEONTOLOGY OF NEW-YORK. 



Fig. 1 A-. An enlarged portion of the margin of the cup, showing that the dentate process be- 

 tween the rays appears externall\-, and extends downwards equally with the full ray. 

 The number of rays usually seen in the cup (except when the dentate process in the 

 margin is visible), is only half the number which will be counted in the outer cir- 

 cumference of the cup. 

 In cases where few and imperfect specimens only are accessible, it is important to ber.r in 

 mind this circumstance, since in this species the rays can be so readily counted on 

 the exterior surface. 



Position and localit;/. In the shale of this group at Lockport, Rochester, Marshall's mill, 

 Wolcott, and almost every other locality where the shale is exposed. {Siau CuiUciion.) 



Genus POLYDILASMA {nov. gen.). 



[Gr. trokvs, multus, 5iS, duplex, and EXatfjAa, lamella ; in allusion to the numerous double lamellae.] 



Corallum turbinate ; lamellae numerous, thin, apparently rising in pairs, and one often much 



stronger than the other ; cell broad, margin thick and strong, with a deep central pit ; half the 



lamellae reaching to the centre of the cell, where they are complicated or contorted ; transverse 



septa in the centre below the base of the central part of the cup, obsolete or irregular. 



This coral is allied to Calopiiyllum, but does not show the transverse septa characteristic of 

 that genus. The cell is very peculiar in its character, one half only of the rays extending into 

 the deeper portion, giving it a peculiar aspect which is preserved in solid crystalline specimens, 

 even when no rays are visible or when they are very indistinct. 



506. 1. POLYDILASMA TURBINATUM. 



Pl. XXXII. Fig. 2a -A. 



Turbinate or clavato-turbinate, usually short ; outer surface nearly smooth, or transversely 

 somewhat rugose ; cell deep, gradually descending from the margin halfway to the centre, and 

 then deepening almost vertically ; rays numerous, thin, becoming complicated before reaching 

 the centre, often apparently crenulate on the weathered exterior ; transverse septa irregular, and 

 sometimes nearly or quite obsolete. 



This species is of variable form, though usually short, turbinate, and rapidly expanding. The 

 cup presents a peculiar feature of being suddenly depressed about halfway from the outer mar- 

 gin to the centre ; and this is likewise accompanied by the termination of one half of the rays 

 at the same point, so that only half the rays of the margin reach the centre. The rays are also 

 variously curved, complicated, or coalescing before reaching the centre. There is no evidence 

 of transverse dissepiments in the cup, and indeed the great depth of the cell in the centre 

 almost precludes their existence in many specimens. The worn exterior surface shows the 

 lamellae in pairs ; and these, on ascending, become again regularly duplicated, so that the 

 inoreaie of lize it pretty uniform on all sides, and ths corallum is usually nearly straight. The 



