Sketch of the Infusoria of the family Bacillaria. 99 



before noticed in that genus. Instead of a snail-like expanding 

 foot, long delicate threads projected where the ribs or transverse 

 marks of the shell join the lateral portion of the ribless lorica, 

 and which the creature voluntarily drew in or extended. An ani- 

 malcule y'g th of a line long, had twenty four for every two plates, 

 or ninety six in the total ; and anteriorly, at its broad frontal por- 

 tion, four were visible. It is probable that this creature may form 

 the type of a special group of the Bacillarias." 



7. Nainctda . (PL 2, fig. 22.) This small species of Navi- 



cula with striate faces, is not uncommon in the infusorial stratum of 

 Richmond, Va. 



b. Without transverse strise. 



8. Navicula . (PL 2, fig. 23, a, b.) This species is distin- 

 guished by having two grooves which cross each other at right angles 

 on the ventral face, presenting a cruciform appearance, and dividing 

 this face into four equal portions, which are without striae. It is a con- 

 spicuous species in many American specimens of fossil fresh-water 

 infusoria, and is very common in the living state. I have found it in 

 New Yoi'k, Ouisconsin and Virginia. 



9. Navicula sigma. (PL 2, fig. 24, a, b.) Smooth, carapace lance- 

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



Our figure represents a sigmoid species, found among marine 

 Algae at Stonington, Conn, A somewhat larger sigmoid species 

 occurs in the infusorial stratum of Richmond, Va. 



10. Navicula J . (PL 2, 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. 6. 



Note. — This may possibly belong to Ehrenberg'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, viz. 



N. alata, 7iov. 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. 



