ISTHMIA. 



451 



I. Isthmie.e. Frustules with valves alike, furnished with 

 a neck-like process. 



GENUS 116. ISTHMIA AG., 1830. 



Frustules compressed trapezoidal, with valves furnished with a 

 neck-like process, usually oblique, cohering irregularly. 



ANALYSIS OF SPECIES. 



Valves furnished with nerves or interior costre 

 Valves without interior costas 



I. nervosa. 

 I. enervis. 



Fig. 175a:. Isthmia Enervis. 

 Girdle view. 



I. enervis Ehr. (inf., p. 209, pi. 16, f. 6 ; h.v.h. Atl., pi. 96, 



f. 1-3*; Type No. 486), plate 19, fig. 625. 



