IMPORT ON THE DIATOMAC1LE. 59 



with the typical and most common form already figured in its zonal and valval aspects 

 by me. 1 It is rare to find a zonal presentation of such a frustule, but the appearance 

 presented by the extremities discovers to the observer that the valve is not linear but 

 inflated, and so probably represents a distinct species. In the absence, however, of more 

 conclusive arguments I have chosen to regard it as possessing nothing more than varietal 

 characteristics. 



Surirella, Turp., Ehrenb., W. Sm. 



Although the recognition of this genus is an easy matter, the determination of its 

 species is a work of great difficulty. The structural details of the Diatoms are constantly 

 found in a more or less developed condition, being sometimes scarcely even outlined, so 

 that it is common to find frustules having the same structural peculiarities, though 

 possessing widely different external contours. 



The genus was first established by Turpin 2 in 1827, and afterwards adopted with 

 modifications by Ehrenberg. Its conception was finally limited by W. Smith, who defined 

 it thus : — " Frustules free, ovate, or elliptical, valves with a longitudinal central line, and 

 margins produced into alas ; canaliculi distinct, usually parallel." But there are two consi- 

 derations which are obstacles in the way of our accepting this definition. Firstly, types 

 with a linear form, but which have all the other principal characteristics, such as Suri- 

 rella linearis, 3 W. Sm., and still more Surirella arctissima, A. S., the valve of which is 

 figured in the Atlas of Schmidt, Plate lvi. figs. 13 and 14, are excluded. The linear form 

 may, however, be looked upon as derived from the oval or elliptical by a process of 

 elongation, while the panduriform outline results from the contraction of an oval form. 

 Secondly, the definition cannot be adapted to some specific types, which, although they 

 exhibit all the other characters of Surirellce, are not provided with a central line. Hence 

 not a few species at present classed as Surirellce should be eliminated, as, for example, the 

 Surirella fastuosa, Ehrenb., var. abludens* Grun., the Surirella intercedens, 5 Grun., and 

 others. This difficulty may readily be overcome by excluding from the definition the 

 clause which demands the presence of a central line. 



Surirella dives, n. sp. (Plate X. fig. 4.) 



Forma elliptica, canaliculis radiantibus, margine denticulatis, a media superficie 

 nascentibus, area centrali arete lanceolata, et lineolarum corona cincta, et medio paucis 

 lineolis transverse signata ; margine striato. Ad Zebu, in mari Philippinarum. 



This beautiful elliptical frustule is noteworthy on account of its well-developed 



1 Atti. Accad. Pontif. d. nuov. Lined, vol. xxxv. sec. 6, May 21, 1882. 



2 Mem. du Mus. d'hist. nat, vol xvi. 



3 Synopsis'of the British DiatomaceaB, vol. i. p. 31, pL viii. fig. 58. 



4 Schmidt's Atlas, pi. six. fig. 1. 5 Schmidt's Atlas, pi. xix. figs. 5 and G. 



