MERISTELLA OF THE HAMILTON GROUP. 805 



tened below the middle, becoming depressed towards the front, which 

 in old shells is produced into a short linguiform extension : umbo 

 gibbous, the beak arching over the umbo of the opposite valve and not 

 closely appressed. 

 DoKSAL valve little longer than wide, varying from moderately convex 

 to gibbous, the greatest convexity being about the middle of its length ; 

 without distinct mesial fold, but abruptly elevated near the anterior 

 margin, corresponding to the depression on the opposite side. 

 Surface smooth, or marked by regular concentric striaa which are some- 

 times crowded into wrinkles near the margins of the valves. The 

 exfoliated shells sometimes show indistinct radiating striae. 

 The muscular impression in the ventral valve is triangular, and usually 

 not deeply marked. The dorsal valve has a distinct median septum which 

 extends nearly half the length of the valve : muscular area narrow, 

 elongate. 



This species presents considerable variety of form, from almost sym- 

 metrically oval to broadly ovate with the greatest width below. The older 

 shells are for the most part gibbous, but some specimens are compressed in 

 the lower half of the length. The mesial sinus is not usually a very dis- 

 tinctive feature in half-grown shells ; but in some individuals it begins 

 about the upper third of the shell, and affects the lower half and ante- 

 rior part of the valve. 



The largest individuals have a length of about one inch and a quarter, 

 with a width of one inch ; in other specimens, a length of one inch and 

 an eighth gives a width of one inch and a depth of three-fourths of an 

 inch. In a gibbous and somewhat elongate form, the depth and width 

 are as 5 to 6, and the length 8 J. 



Figures 5-12, and 16 & 17, represent the most characteristic forms. 



Figurc'B 13 & 14 are of an extreme form. 



Figuie 20 is one of the broader forms. 



Figures 21 & 22, dorsal and front views of a cast of this species. 



Geological formations and localities. This species has been found chiefly in a 

 limestone of the Marcelhis shade, near Le Koy, N. Y. It occurs in the Hamilton 

 group, near York in Livuigstou county, New- York. 

 [ Palaeontology IV.] 39 



