irO PALAEONTOLOGY OF NEW- YORK. 



(T This species is distinguished by the promiftent and rather distant spiniferous 

 tubercles, by the shallow median depression of the Central valve, and also by the 

 Btrong curving spines on the cardino-lateral margins. In an examination of twenty 

 or thirty individuals of this species, I am unable to find any variations or grada- 

 tions that would indicate an approach to any other species. The shell appears to 

 have been very variable in form in its normal condition, and this variation has 

 been increased by pressure during the consolidation of the matrix. The form of 

 the valves does not vary greatly from those of P. hirsutus, but it is usually pro- 

 portionally shorter on the hinge-line. 



The specimens, though occurring in considerable numbers, have not afforded 

 satisfactory means of determining the character of the hinge, or the presence or 

 absence of an area. 



Geological formation and locality. In the Chemung group at Philipsburgh in 

 Allegany county, and near Elmira. 



Prodactclla rarispina. 



PLATE XXIV. 

 Productut rarispinus : Hail in Tenth Report on the State Cabinet, p. 178. 1857. 



Shell semielliptical ; the proportions of length and breadth as 6 to 8, 

 as 7 to 10, or sometimes equal ; hinge-line equalling or a little greater 

 than the width of the shell. 



Ventral valve ventricose, somewhat flattened or slightly depressed in 

 the middle, curving abruptly to the front, and a little more gradually 

 to the baso-lateral margins, abruptly depressed on the cardino-lateral 

 slopes and expanded into ears which are strongly deflected at the 

 extremities. 



Dorsal valve deeply concave in the middle, flattened or slightly concave 

 on the cardinal angles. 



SuBFACE marked by fine concentric striae, and sometimes, in partially 

 exfoliated shells, there are appearances of radiating striae. The ears 

 are strongly wrinkled, and support several comparatively strong round- 

 ed spines, and others of similar character are scattered over the sur- 

 face. These spines rise directly from the shell, Vith a pustule-like 

 elevation on all sides, but not raised in elongate nodes as in most of v. 

 the species. The dorsal valve also bears marks of a few similar spines. 



