50 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 54. 



Pembroke (loc. No. 5.3.8. F), giving in the following table the length, 

 height, and percentage of height to length, and the specific defini- 

 tion under which they fall. 



No. 



Name. 



Per cent. 



M 1215 A. . 

 (Fig. 11).. 

 (Fig. 14).. 

 M1215B.. 



C. 



D.. 



G.. 



F.. 



I... 



K.. 



N. 



T.. 



O.. 



R.. 



S.. 



v.. 

 w. 



Y.. 



E.. 

 M1767L.. 

 M1815.... 



M1817.... 

 M1816.... 



N. corrugatus. 



....do 



....do 



....do 



N. corrugatus. 

 do 



do 



do 



do 



do 



do 



do 



do 



do 



do 



do 



do 



cf. N. corrugatus 



N. pholus 



N. ncssus 



N. ladon 



N. lichas 



52 



62+ 



42J 



42+ 



46 



42 



57 



65 



65 



50 



55J 



52+ 



59 



52+ 



50 



59 



55 



55 



33i 



50 



74 



70+ 



60+ 



It will be seen from the table that 18 specimens were definitely 

 referred to the species N. corrugatus. The average length of these 

 (omitting fractions) is 23 mm. and the average height 53 per cent of 

 the length. The extremes of length are 33 and 15, and 12 of the 18 

 specimens vary but 3 mm. from the mean. The extremes of height 

 are 42 per cent and 65 per cent, and 9 of them (namely, one-half) vary 

 only 4 per cent from the mean, 53 per cent. 



Another specimen is more slender than these (height 38 per cent,) 

 but comes within the average length, 26 mm. 



The other four specimens have a specifically different shape and 

 have been given separate names. 



The shales in which these specimens are imbedded are fine-grained 

 fissile shales, showing no particular deformation since solidification. 

 The specimens do not appear to have been elongated or shortened in 

 any particular relationship to the angle of their position on the shales. 

 Their general surface characters, also, are not so diverse as to furnish 

 basis for good specific distinction. 



It is, therefore, quite consistent with common usage to consider all 

 of them zoologically as belonging to a single species and to regard 

 those named N. pJiolus, N. nessus, N. ladon, and N. licTias as sub- 

 species or varieties of N. corrugatus. 



This interpreti.tion will work no particular harm for purely paleon- 

 tologic purposes, except in making the characters of this species 

 indefinite and elastic; but, from what follows, it wiU be seen that it 

 would be misleading to assume that the variabihty found to be a fact 

 in the fossils represents actual variability of the species in producing 

 its shell, from a zoological point of view. 



