428 DESCEIPTION OF {TolygostAca. 



Tliis species = Navicula Baltica (Ehr.) 1838. — It is distinguislied 

 by the parallelism of its sides, and the great convexity of its valves. 



PLEtTROSTGMA StnffUis. — Valvs lanceolate, uniformly attenuated 

 towards the somewhat acnte extremities; colour paler than the 

 last. Length l-80th. Breadth l-830th. Transverse stria5l-33,000th 

 apart. Hull. Notable for its graceful form and distinct striae 

 (P. 20, f. 4.) 



P. acuminatum. — Valve broadly lanceolate, much attenuated to- 

 wards the extremities; colour a light brown. Length 1-1 62nd. 

 Breadth 1 -1200th. Striae 1-45, 000th apart. It = Kavicula sigma 

 (Ehr.) 1838. Shoreham Harbour. (P. 20, f. 5.) 



P. Fasciola = (Ceratoneis Fasciola, Ehr.) — Yalve lanceolate, ex- 

 tremities produced, flexure considerable ; colour very pale. Length 

 l-240th. Breadth l-1847th. Strife 1-5 5, 000th apart. On the mud 

 of tidal harbom-s (P. 20, f. 6.) 



"I have restored this species to its allies, the elongation of its 

 valves not appearing to rcquii-e its separation, and the more careful 

 examination of its surface showing the central and terminal enlarge- 

 ments of its median line, characteristic of the tme Navieulecey 



P. prolongatum. — ^Yalve much elongated, very narrow, lanceolate ; 

 flexure moderate ; striae obscure ; colour veiy pale. Length 1-1 62nd 

 Breadth l-2400th. Strite imperceptible (P. 20, f. 7.) Poole Bay. 



P. liUorah. — Valve broadly lanceolate, acute; flexure considerable ; 

 longitudinal striae very distinct; colour blueish-grey. Length l-200th. 

 Breadth 1-1 200th. Longitudinal strias 1-1 8,000th apart (P. 20, f. 8, 

 and fig. 19 magnified 5500 diameters.) Coast of Sussex. 



P. Hippocampus ^= Nameula Hippocampus (Ehr.) 1838. — Valve 



broadly lanceolate, obtuse ; flexure considerable ; striae distinct ; 



» 

 colour pale brown. Length l-166th. Breadth 1-1 100th. Strias 



l-36,000th apart (P. 20, f. 9, and fig. 10, a young state.) On muddy 

 shores and bracldsh pools. 



" The specific name of this species has been given to almost every 

 sigmoid Navicula, whether found in the sea or fresh water, by non- 

 scientific observers;" but Mr. Smith would restrict it to the present, 

 although probably the next species was the one to which Ehrenberg 

 originally attached the name. He is also " disposed to believe the 



