90 P. T. CLKVE, SVNOPSIS OF THE NAVICULOII) UIATOMS. 



42. D. didynia Ehb. (1840). — V. .slightly eoiistrieted in the middle, with tongue-.^liaped 

 segments. L. 0,05 to 0,o9; B. O.iu- to 0,036 mm. Central nodule moderately large, its horns not 

 divergent. Fiirrows narrow, linear. Transverse costa; 8 to 10 in 0,oi mm., cro.ssed by numeroiis, 

 slightly iindulating, longitudinal costte. — Piunnlaria didyma Ehb. Kreideth. p. 7ö. Nav. didynia 

 W. Sm. b. D. XVII f. 154 a. A. S. Zeitschr. f. ges. Naturw. 1873 p. 405 Pl. VI f. 1. A. S. 

 N. S. D. Pl. I f. 7. Atl. XIII f. 1, 2, 3. V. H. Syn. p. 90 Pl. IX f. 5, 6. Suppl. B. f. 20. 

 N. JBombus Donk. Pl. VII f. 7 b, 8 h. 



Brackish and marine: Greenland! Spitsbergen! Kära! Finmark! Baltif (from ^\^esterbotten 

 to Rligen), Caspian Sea (Grun.), Black Sea! Ceylon! Tahiti! Japan! Cape Horn (Petit), West Indies' 



D. didynia is a variable species, related to the var. elésdiana of D. s]ilendida and to 

 certain forms of D. Bomius. The outline of the valve becomes less panduriform and almost ellip- 

 tical as the water becomes less salt. The varieties in the northern part of the Baj^ of Bothnia 

 where the water is almo.st fresh are almost elliptical and closely connected with D. domhlittensis. 



43. D. Bonibiis Ehb. (1844). — V. deeply constricted, with .suborbicular or elliptical .seg- 

 ments often of ineqnal size. L. 0,o65 to 0,i.3; B. 0,o22 to 0,o4.'i; at the constr. 0,oi2 to 0,025 mm. 

 Central nodnie large; its horns divergent in the middle, approximate at the ends. Fnrrows narrow, 

 linear. Transverse costa; 5 to 8 in 0,oi mm. crossed by 2 to 5 curved, longitudinal costte. Central 

 alveoli distant from the margin. — Dipl. Bombits Ehb. Berl. 1844 p. 84 (accord. to Chase). M. G. 

 Pl. XIX f 31. Nar. Barnhus Greg. D. of Clyde p. 484 Pl. IX f. 12. Donk. B. D. p. 50 Pl. VII 

 f. 7 a. V. H. Syn. p. 90 Snppl. B f. 22, A. S. Atl. LXIX f. 28, 29. Nav. cjemina A. S. Zeitschr. 

 f. ges. Naturw. 1873 p. 405 Pl. VI f. 2. N. S. 1). Pl. I f. 1, II f. 1. Atl. XIII f. 4, 5, (3, 7, 8, 9. 

 Nav. abnormis Castr. Chall. Exp. XXVIH f. 19. 



Marine: Finmark! North Sea! Maroeco! Mediterranean Sea! Adriatic! Black Sea! Caspian 

 Sea (Grun.)! Madagascar! Java! Japan! Samoa! Galapagos Islands! Cape Horn (Petit), Brazil (Atl.), 

 Florida! Campeachy Bay! Fossil: Aegina (Ehb., Atl.). 



Var. egena A. S. (1875). — L. O, oss to 0,04?; B. O.uiii to 0,02; at the constr. 0,oo6 to 

 0,012 mm. Transverse costne 8 to 9 in 0, 01 mm. crossed on each side of the median line hy about 

 3 longitiidinal costse. — Nav. geniiiia v. cgena Å. S. Atl. XIII f. K). 



Marine: Balearic Islands! Madagascar! Manilla! China! Japan! 



Var. densestriata A. S. (1875). — L. 0, 045 to 0,056; B. 0,oi8 to 0, 026; at the constr. 0, 011 

 to 0,0 15 mm. Costffi 8 to 9 in 0, 01 mm. — Nav. gemina r. doiscstr. A. S. Atl. XIII f. 11, 12. 

 Nav. didyma A. S. Atl. LXIX f. 30. 



Marine: Maroeco! Seychelles (Van Heurck CoU.)! California (Atl.). 



Var. bullafa Cl. — L. 0,15; B. 0, 04 7 in 0, 01 mm. Horns with a row of large puucta. Costse 

 5 to 7 in 0,01 mm. 



Marine: Adriatic! Red Sea (Deby Coll.)! California! 



44. D. Kiitziugii Grun. (1860). — V. strongly constricted, with deltoid-elliptical segments. 

 L. 0,06:5 to 0,15; B. 0,03 to 0,085; at the constr. 0, 014 to 0, 035 mm. Horns of the central nodule 

 parallel. Transverse costii? 6 to 8 in 0, 01 mm., crossed by numerous (8 to 10) longitudinal, almost 

 straight or slightly undulating costcie. — Nav. Kiifzingii Grun. Verh. 1860 p. 532 Pl. III f. 15. 

 A. S. Atl. XIII f. 22, 23, 24. Pant. I Pl. XXIX f. 299. 



Marine: Balearic Islands! Gulf of Naples! Black Sea! Red Sea (Deby Coll.)! Cape of 

 Good Hope! Bermuda (Rae Coll.)! Valparaiso (Atl.), Galapagos Islands! Fossil: Hungary (Pant.), 

 Aegina (Atl.). 



Var. bullata Cl. — Horns with a row of large puncta. 



Marine: Gulf of Naples! Red Sea (Deby Coll.)! 



D. Kiitzingii is very nearly akin to D. Bambus rar. densestriata, but it is larger and tlie 

 horns of the central nodule are parallel. 



