52 NAVICULES. 
leviter constricta, apice late rotundata) ; his figure repre- 
sents it with parallel strig. More recently, in the ‘ Mikro- 
geologie,’ he has given a more definite figure, which re- 
sembles the form selected in the present work to represent 
N. didyma wore closely than any other with which I am 
acquainted. In both of Ehrenberg’s figures the strie are 
parallel, or transverse, and conspicuous. Prof. Smith, in 
the ‘Synopsis,’ figured two widely different species as 
ordinary forms of N. didyma,—one being, as he states, N. 
Apis, Ehren., while the other is evidently the less con- 
stricted form of N. Bombus, Ehren. Rabenhorst, in 1853 
(Siissw. Diat.), appears to have reproduced Ehrenberg’s 
description and figure ; in his recent work, however (Europ. 
Diat.), his description of N. didyma is applicable to N. 
Bombus, Ehren., and certainly not to Ehrenberg’s type of 
the former, which he has reduced to a mere variety (pinnis 
manifestis) ; he has also made N. Apis, Ehren., another 
variety (pinnis subparallelis) . 
These observations will tend to explain the great confu- 
sion with which the present species has been surrounded, 
and the difficulty of ascertaining which is really the original 
Pin. didyma of Ehrenberg. 
Hab. Salt or brackish water: frequent. Coast of Nor- 
thumberland (4. S. D.). 
