REPORT ON THE DIATOMACE.E. 27 



stitnted the genus Pinnularia, are included, but apart from this the species already 

 recorded as belonging to the genus Navicula, properly so called, are so numerous as to 

 tend to bring about confusion in the science, and to render difficult if not distasteful the 

 work of one who introduces new types. Yet such a course is inevitable when a minute 

 examination of marine micro-organisms is made, and it becomes of the greatest importance 

 to subdivide the larger genera, and above all the genus Navicula, distinguishing in them 

 groups based on some common and more salient characteristics, in order to make it possible 

 to remember the different forms. Hence, although recognising the weight of the authority 

 of Kiitzing and Brebisson, who join together the genera Navicula and Pinnularia because 

 some of the latter are found to be granulated, and because there is perhaps no specimen 

 the costules of which, according to these distinguished naturalists, may not be resolved 

 into points, still, as this assertion has not yet been proved, I am unable, resting on the 

 authority of "W. Smith and Eabenhorst, to adopt their arrangement. Still less can I 

 agree with those who join the genus Stauroneis to that of Navicula, although both are 

 naviculoid, because, in that case, many other genera which are likewise constituted on 

 navicular forms would have to be abandoned. 



One of the most extensive group of the genus Navicula is that in which the valves 

 are more or less contracted in the middle and which Ehrenberg designated by the name of 

 Diploneis. 1 In this section the species, on account of their number, are the most difficult 

 to define. 



Navicula abnormis (?), n. sp. (Plate XXVIII. fig. 19.) 



Panduriformis ; apicibus cuneato-rotundatis ; striis subradiantibus moniliformibus ; 

 nodulo centrali quadrato ; area centrali medio subconstricta. 



This singular frustule is remarkable on account of the different magnitudes of the 

 two halves of its valves, and should probably be regarded as an anomalous and monstrous 

 form were it not that several specimens have been found in the same collection. If it 

 should ultimately prove to be a monstrous frustule it will be especially liable to sterility, 

 and although a few generations may come and go its' long survival in the struggle for 

 existence cannot be expected ; it may, however, for the present receive a specific name. 

 Its shape is panduriform, its apices cuneately rounded, and its moniliform striae are 

 subradiating. It possesses a quadrate central nodule, and the central area is slightly 

 constricted at the middle. 



Navicula thaitiana, n. sp. (Plate XX. fig. 5.) 



Bilobata, arete constricta ; lobis subcordatis ; apicibus cuneato acutis ; striis monili- 

 formibus radiantibus, medio evanescentibus ; linea media tenui, nodulis parvis. Ad 

 portum Thaiti. 



1 Pritchard, loc. cit., pp. 892 et seq. 



