DESCRIPTIONS OF SPECIES. 399 



edly triangular shape. The usual outline of the dorsal valve is transversely sub- 

 semicircular, but Meek's specimen agrees with others from the same region in so 

 many ways that I am disposed to regard its peculiarities as of an individual nature 

 and due in part to old age. It is with considerable personal confidence, therefore, 

 that I refer all this material to Si), campestris. 



Meek sought to distinguish /S^. gonionotus from Sp. kentuchyensis and Sj). octo- 

 plicata by reason of certain differences in shape. As the surface ornamentation of 

 the type specimen was removed, he was not able to avail himself of these characters 

 in making comparisons. The differences pointed out by Meek are veritable ones, 

 but if I am not at fault in referring to his species other material from Nevada and 

 adjacent territory, they are not constant and must be referred to individual peculi- 

 arities. On the other hand, on the same supposition, well-preserved specimens 

 among the other material show surface characters which distinguish this species 

 without a trace of doubt from Sj). hentucJcyensis and possibly also from Sj). octoplicata. 

 White identifies specimens originally described as 8p. spinom var. oompestTis with 

 8p. octoplicata. He verj^ correctl}^ regards his material as distinct from Sp. hen- 

 tuckyensis and recognizes its affinities with Sp. spinosa., which, he considers, may be 

 only a variety of Sp. octoplicata. He believes that his specimens differ in lacking 

 the spines which characterize Norwood and Pratten's species. As I have already 

 explained, I believe that their absence is owing to circumstances of preservation. 

 The form from Itaituba, Brazil, which Derbj' identifies with Sp>. spinosa, probably 

 belongs in the synonj-my of White's species. One of his specimens showed very 

 distinctly a rib in the sinus of the ventral valve, a character which he justly remarks 

 is not found in Sp. spin/jsa, nor does it occur in the material of Sp. gonionota which 

 has been studied by me. He further states regarding the surface ornamentation that 

 it shows traces of spines, but not of the regular concentric lamellae characteristic of 

 Sp. spinosa. The latter, however, is evidently a lapsus calami, for this is a character 

 by which Sp>. spinosa is distinctly not characterized. Walcott also identifies his 

 fossils with Spi. spinosa., figuring a specimen from the Coal Measures and one 

 from the Chester to support this view, but he refers Sj). spinosa to x%>. cristata as a 

 direct synonym and places Sp. kentuckyensis Shum. in the same category. It is not 

 unlikely that the identification of this form with Norwood and Pratten's species is 

 correct, but while it superticiallj" resembles Sp). kentuckyensis, the character of the 

 surface ornamentation is so unlike that there can be no question about the propriety 

 of recognizing them as distinct species. The relations of Sp. kentuckyensis and Sp. 

 cristata have been discussed in another place. It seems probable that Sp). cristata 

 does not belong to the spinose section of the genus, a point upon which I have been 

 unable certainly to inform myself; but if it does, and not otherwise, comparisons 

 between Sp. campestris and Sp. cristata will be in order. These, however, I am at 



