86 THE VOYAGE OF H.M.S. CHALLENGER 



the diameter greater, and (2.) in possessing a cylindroidal rather than a cylindrical outline, 

 while (3.) one has the valves exactly hemispherical, whereas in the other more pro- 

 tuberant form they are somewhat globular. Whether such distinctions have more than 

 a varietal significance must at present remain somewhat doubtful, although the evidence 

 would seem to indicate that a specific value may be ultimately assigned to them. 



It may also be noted that the line bounding the upper valve in fig. 12 is undulating, 

 while in fig. 15 it is straight, but provided with three well-marked involutions which are 

 directed towards, the centre of the frustule. The radiating awns in fls;. 12 are also 

 relatively larger and somewhat more elegant than in fig. 14.' 



Corethron, sp. (Plate XV. fig. 7.) 



The organism here represented, which has been figured from a preparation made on 

 board the Challenger, is also manifestly a member of the present genus. In it the 

 awns, which are not represented in full length in the figure, are very strong, long, 

 and ribbed, and — a circumstance which is characteristic of the genus — they are never 

 numerous, while their diameter is almost uniform throughout. 



As this form has not been observed from its zonal aspect its real specific value must 

 remain doubtful. 



Section B. Connecting zone complex, annulate ; awns echinated. 

 Corethron hispidum, n. sp. (Plate XXI. figs. 3 and 5.) 



Forma cylindrica, annulata ; valvis hemisphsericis spinulosis ; setis costatis, echinatis. 

 In mari Antarctico. 



Although the frustules shown in the present figures undoubtedly belong to the same 

 species, in only one are the details of structure represented. The connecting zone is 

 composed of many hoops which are united together, and in this respect, through the 

 genus Lauderia of Cleve, the genus Corethron approaches that of Rhizosolenia. The awns 

 are strong, thorny, and costate, become gradually attenuated from their origin to their 

 distal ends, where they are reduced to mere lines, and spring from the margin of the hemi- 

 spherical valve, the surface of which is roughened by very minute thorns. The presence 

 of these thorns has been employed in naming this interesting species. 



Corethron murrayanum, n. sp. (Plate XXI. fig. 4.) 



Forma cylindrica, annulata ; valvis convexis, la3vibus ; setis costatis, echinatis. In mari 

 Antarctico. 



This somewhat large beautiful cylindrical organism — the longitudinal axis of which is 

 equal to the diameter — has two convex valves, which have the form of the segment of a 

 sphere. The cylindrical median zone is annulated, and a definite line of suture, as in 



