322 



The species under consideration occur in the Gulf Eocene, extending 

 nearly throughout it. They have heretofore been referred to the genus 

 Volutilithes, but are placed by Smith (55) in the genus AthJeta. Smith 

 states that the material at his disposal was very complete, and enabled 

 him to study large series of individuals, very carefully collected with ref- 

 erence to horizon. The stratigraphy of the formations from which they 

 came is also well understood. These favorable conditions of study, it may 

 be remarked, are especially important in the present connection, because 

 they enabled Smith to trace out the evolution of the forms practically 

 continuously from zone to zone, without being chiefly dependent on onto- 

 geny for indications of their relationships. Another fortunate circum- 

 stance is the fact that this author is disposed to use the evidence from 

 ontogeny with the utmost discretion, everywhere checking it by an ap- 

 peal to the morphological and geological series. 



In the forms under consideration, the first two or three whorls are 

 smooth and rounded, constituting the smooth or protoconchal stage. "The 

 first ornamental feature to appear on the smooth, rounded whorl is the 

 transverse rib, that is, a slight elevation of the whorl which runs across it 

 from suture to suture. These early ribs are invariably curved slightly, 

 and each one is simple and uniform from suture to suture. The curved 

 ribs persist as a rule for about a quarter or a half of a whorl, or even 



for a much less space The curved rib stage .... has 



been found in every species and race dealt with in this paper. The curved 

 ribs, after about one-third of a whorl, change abruptly into the straight 

 ribs of what has been designated the cancellated stage." 



"The cancellated condition is found more or less well developed in 

 all the different races. In the primitive races it may persist as a con- 

 stant feature to the end of the individual's life; but in most forms it 

 covers only a few whorls and is more variable than the preceding curved 

 rib stage." The end of the cancellated stage is much less definite than 

 the beginning. It is followed by the "spiny stage." In this stage the 

 shoulder tubercle is sharp and spine-like. Other tubercles have disap- 

 peared, and this portion of the shell is therefore no longer cancellated. 

 Succeeding the spiny stage, there may be a senile stage. 



In the base of the Eocene at Matthew's Landing, Alabama, occurs a 

 species, Athleta limopsis, which from its primitive characters, and its po- 

 sition at the base of the Eocene. Smith regards as the ancestor of the 

 races and species which he deals with in his paper. This form presents 



