380 PALAEONTOLOGY OF NEW-YORK. 



This species presents some variety of fonii, more particularly in the relative 



proportions of the two extremities; st>me specimens being almost symmetrical, while 



others are much narrower on the anterior side. The individuals likewise vary in the 



degree of convexity; and while in well-marked specimens the middle lobe is not 



above the lateral ones, there are others where this lobe rises in a rounded node above 



the plane of the lateral lobes. 



Fig. 3 a. The right valve of a symmetrical form of this species, enlarged four diameters. 



Fig. 3 6. Profile view of the same. 



Fig. 3 c. Figures showing some variety of form, as presented in a group of individuals upon 

 the surface of a fragment of limestone : some of them are maoh more ventricose 

 than others. 



Geological position and locality. In the tentaculite limestone, and base of the 

 shaly limestone of the Lower Helderberg group : Herkimer county. 



This species is often extremely abundant, covering almost the entire surfaces of 

 some layers of the rock. 



The degree of variation in the forms which appear to be traceable to this species 

 induces me to place the following as a variety of the same. 



Beyrichia iiotata, var. ventricosa. 



Plate LXXIX B. Fig. 4 a, b, e. 



Caeapace unequally semielliptical, ventricose, the anterior extremity- 

 much narrower than the posterior : two short furrows mark the sur- 

 face ; the anterior one straight, and shorter than the other ; the po- 

 sterior one longer and slightly curving forward below, with a slight 

 depression around the base of the middle lobe connecting the two 

 grooves. Middle lobe consisting of a small oval tubercle, which does not 

 reach the hinge margin. Posterior lobe narrow above, spreading and 

 swelling out below, becoming quite ventricose and rounded on the ven- 

 tral margin. Anterior lobe smaller and less ventricose, and separated 

 on the ventral side by a slight sinuosity from the posterior lobe. Vep- 

 tral margin incurved beneath the over.spreading lobes, and appearing 

 at the two extremities as a flattened rim, which, on the anterior side, 

 at the junction with the hinge-line, spreads out in a subalate extension, 

 and likewise in a less degree at the postero-cardinal extremity. 



Surface finely granulose. 



