PHYLOGENIC STAGE OF THE GASTROPODA 489 



the spire is reversed, and the aperture is then equally changed 

 from the primitive obliquely backward position to somewhat 

 obliquely forward. On the other hand, if it is vievvcd as a 

 sinistral shell (Fig. 10) related to Scaevogyra, the aperture runs 

 then somewhat obliquely backward, and the narrower or poste- 

 rior side of the aperture is at the umbilical margin (Fig. 1 1), as 

 in that genus, to which the coil also corresponds, being more or 

 less completely convolute {vide M. speciosa Bill.), though the 

 spire's height is rather uniform. From what is known to me of 

 Scaevogyra, its species are considered to be rather greatly vari- 

 able ones, and those of Maclurea, including Maclurina, are also 

 not entirely constant. Even the great number of taxonomic 

 species given by Ulrich {op. cit?) from the Galena is reduced to 

 one species when a large number of specimens are examined. 

 Comparison of immature specimens of Maclurea with Scaevo- 

 gyra will, I think, argue a close relationship between the two 

 genera. 



It is probable that Maclurea descended from Scaevogyra, and 

 that these are nearest to Hypseloconus and Tryblidium. Scaevo- 

 gyra does not occur after the Cambrian time, and, regarding 

 later descendants of Maclurea, none are known, unless they 

 become dextral shells. The mooted transitional forms or linking 

 species between Ophileta and Maclurea in the Calciferous, if 

 ever demonstrable, may be turned to argue the conversion of a 

 primitive sinistral type to dextral Enomphaladae, instead of 

 inversely. The Enomphalidae are, indeed, an ambiguous group, 

 but whether, by reason of diverse origin of species, included in 

 the family or not, does not yet appear. 



Whether the Baraboo gastropods and Maclurea became 

 extinct by dying out, or whether they have converged in part 

 into other taxonomic groups, does not appear. The}- may be 

 related to Recent Gastropoda in the manner of an offshoot 

 from the ancestral fresh-water ancestors, and may be wholly 

 extinct. 



THE ORDOVICIAN GASTROPODA. 



The Ordovician gastropods next succeeding the Calciferous 

 present an increased diversity, which is obviously an advance or 



