52 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 54. 



An average of less than three specimens are found sufficiently alike, 

 morphologically, to be classed under the same definition. Also this 

 diversity of form is shown by the distribution in the faunules. In 

 the faunule 5.4.7 B, five specimens are distributed in three species. 



Faunule 1.45.6 A with only three specimens has three species. 



In faunule 1.43.9 A the 13 specimens fall into 7 species. 



In order to obtain a mathematical expression of this diversity of 

 form, we may take the relation of height to length expressed in per- 

 centages. Taking all the 31 specimens from these splintery shales, 

 sufficiently perfect to give the percentage, the height averages 60 per 

 cent of the length, and the extremes range from 37 to 109 per cent. 



The averages for the specimens of each faunule are 51 per cent, 

 70 per cent, 60 per cent, and 52 per cent. 



The form of the several species as expressed by the percentage of 

 the height to the length is as follows: 



N. galeus, 41 per cent; N. amycus, 44 per cent; N. cf. lentus, 45 per 

 cent; N. cf. corrugatus, 43 per cent (the average for the type is 53 

 per cent); N. pJiolus, 50 per cent; N. cTirysippus, 50 per cent; N. 

 atreus, 59 per cent; N. cf. ladon, 66 per cent; N. battus, 68 per cent; 

 N. tliyestes, 72 per cent; N. euryloclius, 93 per cent; N. pelops, 96 per 

 cent. 



The greatest number of specimens falling under one specific defini- 

 tion is 8 for species N. galeus, the height ratio of which is 41 per cent, 

 with range from 37 to 50 per cent. And the total number of the spec- 

 imens coming within this range of form is 18 or over half of the total 

 number of specimens in the hst. 



From this analysis, it is evident that, whatever may have been the 



original form of the specimens here under consideration, it was a 



narrower, more elongate form than A^. corrugatus, the average height 



of which, in its typical locality (loc. No. 5.3.8 F), is 53 per cent of the 



length. 



METAMORPHIC SPECIES. 



Without going into further details, the evidence is sufficient to 

 show that the species of the splintery shales, all of them, have an 

 entirely different status from ordinary zoological species. 



Independent of the question whether they are well or poorly de- 

 scribed, or as to their taxonomic rank, the causes of their present 

 form are evidently secondary and not (wholly) attributable to the 

 organisms supposed to have produced the shell. 



It is quite evident, also, that these secondary causes have more or 

 less obliterated the original characters. Nevertheless, the characters 

 they now exhibit are as clear and distinct as if they were original 

 characters, and in description and illustration must be treated as any 

 other fossUs. 



