SYSTEMATIC ACCOUNT 281 



lar valves, whose processes were either very much reduced or by reason of the general 

 valve structure rather difficult to define. Cleve's genus is adopted here in the sense in 

 which its author originally conceived it. I do not intend to alter or in any way add to the 

 generic description, but I hope to re-establish the genus by a clearer interpretation of the 

 salient characters of the type species. 



The genus based upon Triceratium arcticam Brightwell is a small and clearly defined 

 one, consisting of about thirty species and varieties. It is composed of what was 

 generally known as the arcticum group of Triceratium Ehrenberg. The cells are poly- 

 gonal in valve view, rectangular in girdle view. The valve surface is slightly convex on 

 concave, and covered with a polygonal areolation which is usually arranged in radiating 

 lines. It is probable that the structure of the areoles differs amongst the species. In the 

 main the areoles appear as loculi or chambers, closed upon the outer surface, but have 

 connection with the interior of the frustule by means of poroids upon the lower wall or 

 floor. The angles of the valves are rounded and seldom much produced, no raised pro- 

 cess or horn of any description appears in the angles. The angles are often a little lower 

 than the central area of the valve, and are furnished with a differentiated area of very 

 fine pores. This area is shaped like a thumb-mark, and exudes the mucilaginous stipe or 

 cushion, by which the frustule attaches itself to the substratum. The valve mantle is 

 deep but never constricted, it is finely areolate. The girdle is always simple, and finely 

 areolate. The girdle is usually as wide as the valve mantle is deep, giving the cell in 

 girdle view the appearance of three equal rectangular zones. The cellulation upon the 

 valve surface is usually entire, but may break down at the centre to form a few isolated 

 puncta. The valve surface is entirely devoid of spines, sulci, canaliculi or costae. 



The genus is littoral. The cells almost invariably begin their existence attached to a 

 substratum, and are solitary in habit. Frequently they are found epiphytic upon algae, 

 sometimes in large numbers. Many spend most of the time as bottom forms, and seldom 

 if ever enter into the plankton. It is rather difficult to define the distribution of the 

 genus, and in some respects it might be said to be almost cosmopolitan. While it has 

 been reported from European waters, it is observed but rarely in the English Channel 

 and North Sea, and at infrequent intervals in the Mediterranean. The genus is more 

 common in the North Atlantic, frequent around the coasts of Greenland, in the Hudson 

 Bay, and the north polar seas. It does not extend southwards below 40 S to any marked 

 degree, particularly in the Atlantic area and associated seas. It is almost entirely absent 

 from tropical Atlantic waters, but occurs frequently, and sometimes in great numbers in 

 tropical Pacific waters, particularly around the Philippine Islands. In the northern 

 Pacific it is common around the coasts of Japan and California. An examination of the 

 plankton from a long line of inshore stations in the Peru current along the west coast of 

 South America failed to reveal any trace of the genus in the tropical and subtropical 

 Zones. In south polar seas, however, the genus is again well represented, and is found 

 frequently associating with the coastal flora of the islands in the South Pacific and 

 Southern Oceans. Owing to the influence of the cold currents moving northwards, 

 some species are found occasionally off the Cape of Good Hope. 



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