46 BRACHIOPODS. 



tity of this shell with P. orbignyi, I arn compelled to differ from 

 Professor Geinitz. I am also satisfied, as elsewhere stated, 

 that the little shell figured by Professor Geinitz under the 

 name P. horridus on his plate iv (Garb, und Dyas in Nebraska) 

 is nothing but a young individual of the species under consid- 

 eration. This shell varies much in the distinctness of its cos- 

 t?e, which are usually rather obscure. It is but necessary to 

 examine a few good specimens to see by their smooth, non- 

 costate umbonal region that they often attain a size even 

 greater than that he has referred to P. horridus, without show- 

 ing the slightest traces of radiating costae. Indeed, some in- 

 dividuals of mature size show but faint indications of ribs 

 even near the front margin, while the various individuals 

 present every intermediate gradation in this character between 

 these and the most distinctly ribbed specimens. In addition 

 to this, the extreme improbability of there being in these rocks 

 a large, conspicuous species like P. liorridus, when no traces 

 of such a shell have ever been seen among all the vast collec- 

 tions that have been obtained from them throughout the great 

 area in which they occur in the West, would alone be a suffi- 

 cient reason for rejecting the conclusion that such a mere mite 

 as this is the young of that species. But the necessity for 

 such an improbable conclusion is entirely removed by the fact 

 that this specimen was found associated with a very common 

 and abundant species, the young of which evidently agrees 

 exactly with it." 



Eegarding the form described by Norwood <& Pratten 

 as Productus muricatus, there is considerable doubt as to its 

 identity with the P. longispinus of the Mississippi valley, 

 although both Meek and Davidson so considered it. Through- 

 out some parts of the continental interior at least, P. muricatus 

 N. & P. presents characteristics that are remarkably constant; 

 and when associated with P. longispinus, no hesitancy what- 

 ever would be entertained in separating the two forms. Were 

 it not for the fact that the name P. muricatus had been used in 

 1836 by Phillips in his Geology of Yorkshire, Norwood & Prat- 

 ten's species would be treated here as distinct from the shell 



