JOHNSTON, ON DIATOMACE^E. 15 



which is also very beautiful; hut while the form and the 

 markings differ, the general characters of upper and lower 

 valves are preserved. 



A. costatus, n. sp. — Front view. — Curved, the superior 

 angles recurved upwards. Both valves transversely costate, 

 each row bisected. Length T |/. 



Side view. — Both valves with rounded extremities; the 

 edges nearly linear. On either side two rows of coarse 

 transverse strise, bounded by a line within the border. The 

 striae on opposite sides alternate, and their points form a 

 zigzag along the median line. The whole surface covered as 

 with a veil of extremely minute puncta. 



The extremities of the upper valve exhibit a radiate dis- 

 position of the bands ; whereas an apparent oval deficiency 

 exists in the lower frustule, but without the inner crescentic 

 line, as in A. angustata. Width Ti5 V?"- (Fig. 14 a, b,f.) 



In Patagonian guano. 



Pleurosigma, Smith. 



P. makron, n. sp. — Has the general characters of P. Bal- 

 ticum, but differs from that diatom in its extraordinary 

 dimensions, its conspicuous convexity longitudinally on either 

 side of the keel, the coarseness of its markings, and the 

 attendant colour, dusky olive. Length 4 V' ; width 3 ^ T " ; 

 dots 30 in 0-001" longitudinally, and 33 in 0001" transversely. 



Pongateague, Virginia. 



P. . — Probably a variety only of P. Balticum. Length 



4 \" ; width sh"; dots 36 in 0-001". Frustules slightly 

 swollen near the extremities ; and is of a deeper reddish-brown 

 than its congener. 



Pongateague, Virginia. 



I have had no opportunity of examining the diatoms of the 

 lower part of the Chesapeake Bay, except as derived from 

 oysters cultivated there. They occur, however, in rich variety 

 and profusion, embracing many beautiful species of Nitzschia, 

 Navicula, Pleurosigma, Coscinodiscus, &c. But, from an 

 inspection of a considerable number of prepared valves (always 

 separate and viewed laterally) , I think that P. makron may be 

 safely admitted as a new species ; for although there is some 

 little variation as to size, the appearance of the valves, their 

 peculiar hue, and the distance separating the dots, remain 

 constant. The characters are, consequently, tranches. 



With regard to P. , there are, to distinguish it 



from P. Balticum — 1st, the length 4 V, while the latter is 

 but T y by my own measurement j 2d, the turgidity of the 



