386 DESCRIPTION OF [Pol^ffastrica- 



This genus has the habit of BiddnlpJiia, but is devoid of the lateral 

 constrictions. It has the form of a Pan's pipe, with two orifices on 

 one side. 



Hemiaulus antarcticus. — Sub. quadra to, strongly granular, tridendatc 

 on each side ; the central dentation shallow and obtuse — the lateral 

 ones longer ; two truncate and two opposite ones aciuninate. Dia- 

 meter 1-3 14th. (P. 14, f. 54.) 



H. (?) australis, — Strongly granular, tridentate on each side ; the 

 central dentations very small (obsolete) ; the lateral rounded. Dia- 

 meter l-llo2nd. 



Genus Hemipttchtjs, (Ehr.) — Lorica composed of two equal 

 valves, orbicular, not in chains (?). Its interior divided by imperfect 

 septa, extending about half way towards the centre, and having no 

 depressions on the surface, alternating wdth them ; disc without 

 markings, but cro-^Tied by a zone of teeth (denticuli;) apertures on 

 the margin not evident. 



This genus diffei's from Actinajyti/cltus by its imperfect septa, by 

 the absence of alternating depressions, by the circlet about the centre, 

 and by the marginal apertures being obsolete. It seems, by its des- 

 cription, to be almost identical with Arachnoidisciis of English 

 naturalists. ' 



H. ornatus. — Disc minutely granular ; twenty-nine equal rays, and 

 an intervening concentric apparatus of cells. Diameter 1-1 20th. 

 Found in Patagonian guano. 



The individuals of this species occur as comparatively very large, 

 thin, discoid plates, which exhibit radii upon their surface, connected 

 by a very delicate net work, after the manner of the genus Actinop- 

 tychus. The radii are like"\vise raised bands, and extend from the 

 margin towards the centre, but do not reach it, leaving a broad cen- 

 tral disc, traversed by finely-dotted radiating lines, terminating at 

 the circlet of teeth at the centre. 



Genus Hemizostee, (Ehr.) — Eound siliceous tubules, occurring as 

 so many half circles, contiguous and striated. 



" I (says Ehrcnberg) have contrived this name, to keep in mind 

 certaiu singular but not uncommon corpuscles. Are some associated 

 corpuscles, like Pyxidicula, the lateral (Hscs of these forms? " 



H. tubulosus. — Testule cylindrical, two to three times longer than 

 broad, tiu-gid, trausversel}- annular, resembling a portion of a trachea 



