TRENTON LIMESTONE. 



177 



228. 16. PLEUROTOMARIA PERCARINATA (n. s/).). 



Pi.. XXXVnr. Fig. 1. 



Somewhat obtusely conical, vontricose ; spire short, ohtiise ; volutions three or four, 

 rounded, marked by numerous spiral carinee, which are crossed by vertical or undulating 

 striae ; aperture not distinclly visible, but apparently it is broadly oval or rounded ; 

 umbilicus none. 



This species is readily distinguished from all tlie others known in this rock, by the 

 numerous spiral elevated ridges or carina; which mark tiie surface. It approaches very 

 nearly in character to the Pleurotomaria hilix of Conrad {Joxir. Jlcad. J\~at. Sciences, Vol. 

 viii. pag. 211, pi. 16, fig. 10); but differs from that shell in being more ventricose, and 

 in the volutions being rounded above, while in that species they are flattened, and abruptly 

 contracted below, giving it a more trochiform aspect. 



This species and the preceding one differ from Pleurotomaria in the form of the 

 aperture, and approach in this respect to Murchisonia. 



Position and locality. In the compact blue limestone, upper part of the Trenton limestone. 

 Middleville. , , . 



229. 6. MURCHISONIA BICINCTA. 

 Pl. XXXVIII. Figs. 5a-h. 

 Compare Pleurotomaria angtilata, Sowerbt in Sil. Researches, pag. 641, pl. 21, fig. 20. 



Obliquely subconical ; spire elevated, acute ; volutions four or five, angular, rapidly 

 enlarging towards the aperture ; last one ventricose below, tricarinatc, the lower carina 

 hidden by the suture of the next volution at tlie upper inner angle of the aperture ; central 

 carina on the outer angle of the volution, margined on either side by a sharp elevated 

 line, with a narrow groove between, producing a double spiral band ; aperture oblong, 

 angulated below ; surface marked by fine sharp striae, which bend gently backw^ards, and 

 are but slightly undulated in passing the first carina, from whence they turn more suddenly 

 backwards to the mesial band, making an abrupt retral angle, and then bending forwards 

 below, pass in a vertical direction to the suture. In the last volution, the striw pass vertically 

 to the lower slight carina which corresponds with the suture in the other volutions, and 

 from thence bend slightly backwards, curving into the umbilicus. 



The minute description here given will be found perfectly applicable to entire and un- 

 worn specimens ; but it is often found in fragments and casts, with the surface markings 

 more or less obliterated. The doui)le spiral band becomes obsolete, and only a single ridge 

 is manifest ; the lower carina on the last volution is not visible in casts, and there is but 

 an obscure indication of the upper one. The entire casts, therefore, present scarcely more 

 than the single marginal angle, indicating the direction of the mesial band, and, in this 

 respect, correspond with P. migvlafa cited above ; Inu tlie volutions in our shell are more 

 ventricose. 



I Pai,.e()NT(ii.ogy.| 23 



