OF THE FAMILY BACILLARIA. 147 



9. Navicula Sigma. (PI. 11, fig. 24, a, b.) Smooth, carapace lan- 

 ceolate, sigmoid, not striate, linear, lanceolate on the straight side. 



Our figure represents a sigmoid species, found among marine 

 Algai at Stonington, Conn. A somewhat larger sigmoid species 

 occurs in the infusorial stratum of Richmond, Va. 



10. Naricida? . (PI. II, fig. 25, a, b.) This very remark- 

 able form I detected among fossil infusoria, from the infusorial stratum 

 of Richmond, Va. It is lanceolate when seen on one side ; on the 

 other side it presents the curious outline shown in fig. b. 



Note. — This may possibly belong to Ehrenb^rg's new genus 

 Zygoceros, which is described as having a compressed Navicula- 

 shaped carapace ; each end provided with two perforated horns. 

 (See Pritchard, 1. c. p. 427.)* 



In addition to the American species of Navicula above de- 

 scribed, Ehrenberg mentions the following as occurring in a fossil 

 state at West Point, namely : 



N. alata, nov. sp. 



N. amphyoxys. 



N. Suecica. 

 I am, however, ignorant of their specific characters ; I have 

 met with many species besides those referred to in the present 

 memoir, but omit them, as my present object is to present only 

 the most interesting forms. 



EUNOTIA. 



Free, single or binary, carapace simple, bivalve or multivalve 

 {siliceous) prismatic, four openings on the same side, tivo at each 

 end, ventral side fattened, back convex and often dentate, never 

 catenate by perfect spontaneous division. 



1. Eimotia arcus. (PL II, fig. 26, a, b.) Striate, carapace semi- 

 lanceolate, elongated, two terminal knobs arcuate, eleven strias in 

 -j-fft) of ^ line. 



* Since this memoir was read, I have venUired to refer this species (PI. II, fig. 25, a, b.) 

 to a new Genus Emersonia founded on this, and a very beautiful living species which I 

 detected in Boston harbor. The genus is named in honor of G. B. Emerson, Esq., Presi- 

 dent of the Boston Natural History Society. Vid. Boston Journ. Nat. Hist., Vol. IV. 



