No. 109.] 173 



unlike the variety of P. undiferus figured bjDE Koninck [Recherchcs sur 

 les Animaux fossiles, pi. si, f. .5 a, b, c ) ; but the hinge-line is less ex- 

 tended, and the surface exhibits no regularly radiating costas as in that 

 species. 



Geological position and locality. Shales of the Hamilton group : Mos- 

 cow, N.Y. 



Productus spinulicostjs (n. s.). 



Pal. N.Y. Vol. iv. 



Shell semiorbicular, width and tength about equal; hinge-line 

 generally a little less than the greatest width of the shell : dor- 

 sal valve marked with spiniferous ridges, which are sometimes 

 continuous from the middle to the base of the shell : ventral 

 valve extremely gibbous in the middle ; beak incurved. Surface 

 marked by fine concentric lines, which are sometimes crowded 

 and wrinkled on the body of the shell, ornamented with several 

 rows of short radiating interrupted ridges, each one of which 

 is furnished with a small spine : a row of four or five spines 

 along the hinge-line below the margin, which are often continued 

 along the sides of the shell, and sometimes along the front, 

 entirely or partially independent of the spiniferous ridges. 



The distinguishing characters of this species are the nearly semiorbicular 

 form, and somewhat regular alternating distribution of the radiating 

 elongated spiniferous tubercles. The length and breadth is from one-half to 

 three-fourths of an inch. •; 



This species bears some resemblance to the smaller forms of P. mur- 

 chisonianus given by de Koninck (pi. xvi, f. 3 c^, e) ; but I regard it as 

 distinct. The Strophomena memhranacea of Vanuxem, referred by 

 DE Koninck to the same species, is entirely distinct from the one under 

 consideration. The figures of de Verneuil (Kussia and the Ural Moun- 

 tains, Vol. ii, pi. xviii, f. 4 ) are much more like a species found in the 

 shales and limestones of the Hamilton group in Illinois, Missouri, etc., 

 and which I regard as quite distinct from those of New-York. The present 

 species bears some resemblance to fig. 99, pi. xxv, Phillips, Pal. Fos- 

 sils, but not to the other figures of that author cited by de Koninck. 



Geological position and locality. In calcareous bands in the HamiltoB 

 group : Shores of Cayuga lake, etc. 



