LOWER HELDERBERG ROCKS. 381 



Fig. 4 a. The right valve. Fig. 4 6. The left valve. Fig. 4 c. Profile view. 



These figures are enlarged to four diameters. 



Geological position and locality. In the shaly limestone of the Lower Heldei-berg 

 group : Herkimer county. 



Beyi'icliia trisulcata ( n. s.). 



Plate LXXIX B. Fig. 5 a, b. 



Carapace minute, semioval ; anterior extremity scarcely narrower than 

 the posterior ; marked by three transverse furrows which are abruptly 

 impressed, the central one being essentially central to the entire valve, 

 shorter and broader than either of the others, and very slightly bending 

 at its base towards the posterior end. Anterior and posterior furrows 

 rectangular to the hinge-line, and reaching nearly to the ventral 

 margin ; the anterior one reaching more nearly to the ventral margin, 

 and curving slightly backwards at its base. The posterior median lobe 

 narrower than the anterior median one, and a little more elevated ; the 

 anterior lobe narrower than either of the others, declining towards the 

 antero-cardinal margin, which is flattened. 



ScEFACE very finely granulose. 



This species is distinguished from any other in this group, by its minute size, 

 and by the three transverse furrows, the central one of which is conspicuously 

 shorter. In some of the specimens there appears to be a slight variation from the 

 proportions of the parts as described, but I have thus far observed no essential 

 differences. 



Fig. 5 a. A specimen : diameter eight times enlarged. 

 Fig. 5 b. Profile view of the same. 



Geological position and locality. In the tentaculite limestone of the Lower Hel- 

 derberg group : Herkimer county. 



