130 THE VOYAGE OF H.M.S. CHALLENGER. 



la Pritchard's History of the Infusoria (p. 841), we read: — "All the species of 

 Omphalopclta resemble Actinoptychus senarius," and indeed all the specific forms there 

 recorded have six compartments and the characteristic submarginal spines. The Diatom 

 now in question, however, forms a beautiful disc, with eighteen compartments, each of 

 which is furnished with a distinct denticule or spine. As in all the species of Omphalo- 

 pelta, the segments are alternately elevated and depressed, while the centre is occupied 

 by a smooth ill-defined area. The axis of each segment is marked by a radial line, which 

 extends from the central area and disappears soon after reaching the middle of the com- 

 partment. At the circumference, and at the middle of the outer margin of each compart- 

 ment, there is an evident denticule which is of the utmost importance for the generic 

 determination. The valve is ornamented with small decussately disposed granules, while 

 traces of areolation are found here and there. 



The frustule presents a great resemblance to a Diatom found abundantly by Shadbolt 1 

 in guano from Callao, as well as in the Port Natal gathering, upon which that observer 

 instituted the genus Actinophcenia, and which he named Actinophcenia splendens. It is 

 hard to understand the reason which induced Pritchard 2 to class this form with the genus 

 Actinoptychus, since he described it as possessing a denticule or spine in each compart- 

 ment, and so should have named it Omphalopelta splendens. On comparing, however, the 

 description with our preseut figure, it may be observed that in Sbadbolt's species the 

 umbilical hyaline area, instead of being indefinite, is well defined. It is further to be 

 noted, on consulting the figures of Actinophcenia splendens given by Eoper, 3 that the 

 denticulcs correspond to the jjoint of division between two compartments. Thus in 

 Pritchard's supposed Actinoptychus, the segments would not be alternate in the ordinary 

 sense, we should rather have to regard the number of the compartments as being double 

 what is stated, and to look upon the denticules not as placed at the middle of the segments 

 but as marking the extremities of the divisional lines. Moreover, in our present frustule, 

 the number of denticules is greater than in that of Shadbolt, and the vertices of the elevated 

 segments are narrower than those of the depressed segments. Finally, Shadbolt's species 

 is destitute of the striated margin seen in the present case. From such considerations 

 the specific value of the present form from the Sea of Japan can hardly be questioned. 



Omphalopelta (?), sp. (?) (Plate XVI. fig. 8.) 



This small irregular disc is probably a monstrous form of some species of Omphalo- 

 pelta. The cuneate septa are almost invisible, while the outline of the valve is entirely 

 anomalous. Like other species of this genus, however, it possesses a hyaline umbilical 



1 See a note entitled A defence of the proposed new genus Actinophcenia, Shadb., in Micr. Journ., vol. ii. 

 pp. 201-203; Edwards on Diatomaceas collected in the United States, Micr. Journ., vol. vii. 1859, p. 88. 



2 Pritchard, op. cit., p. 840. 



3 Eoper on the Diatomaceae of the Thames, Micr. Journ., voL ii. pp. 73, 74, pi vi. fig. 2. 



