74 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 148 



ent horizons are available. Each thin limestone bed or lens, where the 

 well-preserved material occurs, shows a population with a certain 

 range of variability, but often statistically distinguishable from those 

 occurring in lower or higher beds. In certain cases the range of varia- 

 tion within a population is such that extreme individuals overlap in 

 several features those of different populations. Nevertheless, as mate- 

 rial from a larger number of localities was studied, 17 well-character- 

 ized species emerged. Most of these could be found at a number of 

 different localities in the same stratigraphic order, hence are valuable 

 for correlation. 



A favorable factor in the study of this difficult genus was offered 

 by the frequent occurrence in a single bed or lens of a coquina of 

 fragments of a single species. Hence the various parts could be 

 assigned unequivocally for all the species described herein. Less fre- 

 quently 2 species occurred in the same bed, and only 1 association of 3 

 species (A. camiro, laxa, and quadrata) was observed. 



Difficulties in identification resulted from the presence in some beds 

 of only immature forms of species that occur in much larger sizes 

 at other localities. Cranidia less than 4 or 5 mm. in length are often 

 indeterminable. Exfoliated specimens are difficult to compare accu- 

 rately with individuals preserving the test; the latter were chosen 

 almost exclusively for the descriptions and illustrations. Material pre- 

 served in shale or siltstone can seldom be specifically identified. 



Virtually all the specimens collected could be assigned to the species 

 discussed herein. However, it is to be expected that future collections 

 from new localities will yield further new species, owing to the fact 

 that fossiliferous limestone beds in shale may occur at slightly different 

 horizons at different places. 



In agreement with Palmer (1962b), Proaulacopleura Kobayashi, 

 1936, C lev eland ella Resser, 1938, and Labiostria Palmer, 1954, are 

 here considered subjective synonyms of Aphelaspis. 



The characters of the genus have been described in great detail by 

 Palmer (1954, 1962b). Only one point concerning ornamentation will 

 be mentioned here. The upper surface of the test in the southern Ap- 

 palachian species is either perfectly smooth, or finely punctate, in 

 .some species only in the furrows {A. inerniis, A. arsoides, A. pal- 

 meri). The inner surface is invariably very finely granulate, and the 

 internal impression therefore finely and densely pitted. Lack of com- 

 ment on ornamentation in the specific descriptions means that the 

 outer surface is entirely smooth. 



The characters most frequently used for specific discrimination are 

 the following. 



