CONTRIBUTIONS TO PALAEONTOLOGY. 



191 



MERISTELLA QUADRICOSTATA. 



Orthis quadricostata : Vanuxem, Report on 3d Geol. Dist. New-York, 1843, p. 168. 

 Atrypa quadricostata : Hall, Report on 4th Geol. Dist. New-York, p. 223, f. 2. 



Shell thin, flattened, transverse, marked by distinct rounded plications in the 



middle of the valves; lateral portions plain, or with faintly marked plications; 



without distinct mesial fold or sinus. 



This species is smaller than the preceding, with fewer radiating plications. It 

 differs from M. multicosta in the absence of a mesial fold, and smaller number of 

 ribs. 



Geological formation and locality. Upper black shales of the Hamilton group: 

 Bigstream point, Seneca county; and other places in Western New-York. 



MERISTELLA MESACOSTALIS. 



Atrypa mesacostalis : Hall, Report on 4th Geol. Dist. New-York. Illustrations of Fossils of 

 the Chemung group, G4, ff. 1, 1 o, 1 b. 



Shell somewhat elongated, with moderately prominent beak. Valves convex, with 

 well-defined mesial lobe and sinus, which are covered with plications; those 

 bordering the sinus are the largest. Lateral portions of the shell smooth, or with 

 obscure ribs. 

 This species is usually larger than any of the preceding, and is characterized by 



its elongate form and plain or obscurely marked lateral portions of the shell. 

 Geological formation and locality. In rocks of the Chemung group : Steuben 



county, and other parts of Western New-Y^ork. 



lOf 



MERISTELLA MESACOSTALIS. 



Fig. 1,1a. Ventral and cardinal views of full-grown individuals of M. mesacostalis. 

 Fig. 1 h. Ventral view of a young individual of the same species. 



