10 Rev. W. Smith on the Diatomaceae, with descriptions 



Very like the last, but much more gradually attenuated to the 

 extremities : the striae are imperceptible. 



Plate II. fig. 7. Valve of P. prolongatum. 



14. Pleurosigma littorale, n. sp. Valve broadly lanceolate, acute; 

 flexure considerable ; longitudinal striae very distinct : colour 

 bluish gray. Average length of valve s ^ of an inch, greatest 

 breadth of ditto j^. Long, striae la ^ 00 of an inch apart 

 (v. v.) . 



Coast of Sussex, 1850. 



A very distinct and curious species. The valves in the broadest 

 part are marked with about fourteen longitudinal striae, which 

 can be resolved by a moderate power into rows of beads about 

 ^oUoo °f an mcn a P ar t« When the light is thrown upon the 

 object so as to exhibit these beads as transverse striae, the sur- 

 face appears marked with a succession of short lines or furrows 

 crossing the longitudinal striae, which still continue visible, and 

 the frustule assumes a very peculiar and beautiful appearance. 

 These effects may perhaps be owing to the circumstance, thus 

 mentioned by Mr. Kingsley : " There are beads on both sides 

 of this valve, and the distances are not the same for both sides, 

 so that with oblique light and imperfect definition, the two sets 

 in some degree obliterate each other and cause a confusion in the 

 image." 



Plate II. fig. 8. Valve of P. littorale; fig. 19. portion of same magnified 

 5500 diameters (Rev. W. Kingsley). 



15. Pleurosigma Hippocampus, mihi. Valve broadly lanceolate, 

 obtuse; flexure considerable ; striae distinct : colour pale brown. 

 Average length of valve T ^ of an inch, greatest breadth of 

 ditto jjqq of an inch. Striae ^^oo of an inch apart (v. v.). 



Navicula Hippocampus, Ehr., Pritchard, Hist, of Animal, pi. 3. 

 fig. 145 ; Kiitz. Bacill. p. 102. tab. 4. fig. 29 ; Phy. Ger. p. 95 ; 

 Sp. Alg. p. 86. 



On muddy shores and in brackish pools. Lewes, 1850. Hull, 

 Mr. R. Harrison ! 



The specific name of this species has been given to almost 

 every sigmoid Navicula which has fallen under the notice of the 

 non-scientific observer, whether found in the sea or fresh water. 

 It seems however desirable to confine a term borrowed from a 

 marine object to a species of a similar habitat, and I have there- 

 fore adopted it for the present, although it is probable that 

 the next species was the one to which Ehrenberg originally 

 attached the name, as it certainly is the one usually esteemed the 



