BR1GHTWELL, ON DIATOMACE.E. 93 



utricle with the contents, and the formation round it at the 

 ends of new cell-wall, how an articulation could exist to allow 

 of the ultimate separation into two fronds of the old segments 

 with the portion that should appertain to each of the newly- 

 grown structure. 



What causes the change in the direction of the axis of 

 growth in the specimens represented by some of the figures, 

 does not appear to me so readily to be accounted for. In 

 each case I should suppose the plant woidd cease to grow, 

 and the abnormal individual perish, unless, indeed, each or 

 any might be supposed to possess the power of afterwards 

 forming a septum at the suture connecting the newly-grown 

 portion with the older segments, a second new growth 

 becoming then interposed, and the central misshapen structure 

 thus becoming eliminated. 



On some of the Rarer or Undescribed Species of Diato- 

 mace.e. Part II. By T. Brightwell, F.L.S. 



16. Actinocyclus areolatus. — Xalve with a single spine or 

 projection on the upper margin of each area of the disc. 

 Varies in size from -0:23 to -013. (Plate V, fig. la, lb.) 

 The latter an assumed diagram of a front view. 



Omphalopelta areolata, Ehr. — In shell cleanings and guano 

 not unfrequent. 



Closely allied to A. undulatus. — Mr. T. West says the 

 spines or projections are often seen on alternate areas of the 

 disc only. 



17. Actinocyclus trilingulatus. — Valves very convex, divided 

 into six segments, alternately elevated and depressed. The 

 elevated segments gradually rise from the circumference to 

 near the centre, where they are rounded off; each alternate 

 segment has a sub -marginal row of dots, or truncated pro- 

 cesses. Surface delicately punctato-striate, -035 to '073. 



(PI V, fig. 2; 2a, front view ; 2b, front view, in self- 

 division.) Of this large and beautiful species a few specimens 

 have been found in shell cleanings, West Indies. 



18. Actinocyclus spinosus, n. sp. — Convex, valves of six 

 segments, alternately slightly elevated and depressed ; a few 



