1006.] NATURAL SCIENCES OF PHILADELPHIA. 53 



The material at my disposal is very full and complete,^ and has 

 enabled me to study large series of individuals. These series in each 

 case represent the forms in the proportions in which they occur in the 

 field, and therefore offer every opportunity for obtaining the average 

 racial characters in any particular locality or horizon. 



Morphology. 



The genus Volutilithes is widely distributed throughout our Southern 

 Eocene. At some localities several sharp and distinct species occur, 

 but at most of the localities Volutilithes is represented by one species 

 and by a particular race of that species. The term race is used in place 

 of variety, for in these assemblages the individuals frequently differ 

 widely. There is, however, always an average of characters which 

 stamps the stage of evolution which the race has attained. Of course, 

 extreme specimens often have more of the features of some other race 

 than of the race to which they belong, but these individuals are always 

 few; and there is never any difficulty in recognizing the general race 

 characters for any particular geographical point, and the phase of 

 phylogenetic development found in the faunule at that point. In 

 cases of this sort, the introduction of varietal and specific names is to 

 be deprecated, and the adoption of the system of race analysis is 

 strongly urged by the author. 



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

 smooth and rounded, constituting the Smooth Stage. The first orna- 

 mental feature to appear on the smooth, rounded whorl is the trans- 

 verse rib, that is, a slight elevation on 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; in fact, sometimes we have only one or two of them. 

 This Curved Rib Stage is short but remarkably constant, and though 

 occasionally much suppressed, 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 * 



' It forms part of the Isaac Lea Collection of Eocene MoUusca at the Academy 

 of Natural Sciences of Philadelphia, and was brought together largely by Mr. 

 Charles W. Johnson, now of the Boston Society of Natural History. The col- 

 lections are a tribute to his skill and perseverance as a collector, and are an example 

 of what museum research collections should be. 



* See Dall, Trans. Wag. Inst., Vol. Ill, p. 68. 



