86 [Senate 



LEIORHYNCHUS 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 X. multicosta in tlje 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. 



LEIORHYNCHUS MESACOSTALIS. 



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

 of the Chemung group, 64, ff. 1 a, 1 &. 



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-York. ^ 



1 



LEIORHYNCHUS MESACOSTALIS. 



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

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



PENTAMERUS PAPILIONENSIS. 



Pentamerus papilionensis, cited in Geol. Report of Iowa, 1858, p. 514. 

 Shell ventricose, broadly ovate. Ventral valve the most convex : beak obtuse, 

 incurved; a broad mesial sinus reaching nearly to the beak, and produced in 

 front in a linguiform extension; surface marked by few or many rounded or 

 subangalar unequal plications, of which, three, four, or five occupy the sinus, 

 with sometimes five, six or seven on each side. Dorsal valve gibbous, with a 

 prominent mesial elevation which is marked by several plications. The triangular 

 foramen, or opening to the V-shaped process of the ventral valve, is visible above 



