401 DE8CKIPTI0N OF {JPolygostnca, 



aperture not present; striiB narrow, twenty-four in 1-1 150th. 

 Leiigtli 1 -540th. 



SuKiKELLA Gemma. — Ovato-oblong, hxrge, turgid, central aperture 

 not present ; striaj slender, sixteen in 1 -1 150th. Alive at tlie mouth 

 of the Elbe, and in various tidal harbours of England. Length 

 l-290th. to 1 -220th. 



S. Gemma. — Mr. Smith says — " Erustules on front view wedge- 

 shaped, with rounded ends ; side view ovate-elliptical ; alae large ; 

 costse small, unequally distant ; surfiice of valve distinctly striated. 

 The striae are made out with difficulty on the dry valve after burning 

 or maceration in acid. Its costfe are linear, unlike those of S. striata, 

 craticula, constricta, &c. (P. 15, f. 2, 3, and 4.) 



" It was in connection with this species, that Ehrenberg records 

 the presence of cilia, extending from the aperture of the costae, 

 vibrating with rapidity, and being extended or retracted at intervals. 

 (P. 15, f. 3 and 4.) The presence of delicate hairs, apparently on 

 all parts of the frustule, may often be detected ; and I have noticed 

 them on nearly every occasion when I have gathered this species, but 

 in no case have I been able to perceive any motion in such hairs, 

 and concluded, before meeting -ndth Ehrenberg's remarks, that they 

 were merely a parasitic growth, the mycelium of some other algae. 

 I have noticed similar appendages to other Biatomacece, but in every 

 case devoid of motion." 



S. Clypeiis. — Large, ovate, obtuse, with nine very broad pinnae in 

 l-276th. Marine. Length l-276th. 



S. craticula. — Lanceolate, with large aloe ; apices on the dorsal 

 aspect acute; on the lateral, truncate; and figure oblong, with 

 centre slightly inflated; pinnules seven in l-1200th. Length 

 l-288th; smaller than S. hifrons; costoe fewer, and those divergent. 

 (P. 15. f. 19 and 20.) 



S. fastuosa. — Larger, elliptic ; alae small ; pinnules dilated 

 gradually, eighteen on each side, in l-360th, which is its length ; 

 central portion of valves smooth ; apertures of costal tubes large. 



S. lamella. — Large, lamellar, ovate-lanceolate, slightly keeled ; 

 striated only on its extreme margin, the whole central space being 

 granular; laterally narrowly linear, and truncate. Length l-216th. 

 to 1-1 80th. 



