1 6 STUDIES IN GEOLOGY, No. 3 



species, notably from the Floridian Tertiary, are differently 

 developed. 



Phos latirugatus represents a type which is apparently 

 little developed anywhere in known faunas; most of the 

 Photina-e have more rounded, less sharply carinate whorls, 

 and members of the type here found, with a distinct 

 shoulder, are not common. 



The pelecypods offer somewhat more widely distributed 

 diversity than do the gastropods, although they, too, are 

 unbalanced on account of their incompleteness, as a result 

 of which statistical biologic discussion can enter into few 

 features with profit. The species are distributed through 20 

 genera and 13 families. The Taxodonts are an important 

 element, being represented by 20 species and varieties, of 

 which 19 are of the genus Area. These 19 species and var- 

 ieties, distributed through 4 subgenera and sections, are the 

 most brilliant and prolific, if not the most interesting of the 

 Zorritos pelecypods. The Lower Zorritos group is charac- 

 terized by smaller species, and the Upper Zorritos and 

 Variegated by larger, heavier forms. The closely related 

 groups have been difficult to separate satisfactorily, and the 

 author is fully conscious of the fact that others may view 

 the present treatment with thoughts of revision; however 

 the grouping here offered is believed to be the most rational 

 according to the more or less unexpressed limitations of 

 morphologic differentiation, and the amount of material 

 available for use in discriminating some of the more trouble- 

 some species has been sufficient to give the author some 

 measure of confidence in his decisions. 



It is apparent that the .Zorritos region, as well as the 

 Antilles and Florida, was the scene of the divergence of the 

 Cunearcas from the true Scapharcas ; the presence of three 

 species, Area larkinii Nelson, Area pantheonensis n. sp., and 

 Area zorritensis n. sp., very near the border-line between 

 the groups, each exhibiting some feature either somewhat 

 anomalous to the preponderance of other sectional char- 



