392 HYALONEMA (THALLONEMA) GEMINATUM. 



Among the amphidiscs two main kinds can be distinguished morphologi- 

 cally: — larger ones with relatively large anchors, and smaller ones with relatively 

 medium-sized or small anchors. I consider the former as macramphidiscs, the 

 latter as micramphidiscs. 



Among the macramphidiscs three subgroups can be distinguished morpho- 

 logically : — a, large ones with short and broad anchors, some to most of the 

 teeth of which are branched; b, middle-sized ones with short and broad anchors, 

 and simple teeth; and c, smaller ones with long and broad anchors and simple 

 teeth. These three macramphidisc groups are connected by transitional forms 

 both morphologically and biometrically. The morphological connections be- 

 tween b and c are macramphidiscs under middle size with anchors of medium 

 length. The morphological connections between a and b are large macramphi- 

 discs in which only one or two anchor-teeth are branched. 



As the length frequency-curve pertaining to the macramphidiscs in Figure 26 

 shows, there is a conspicuous enough depression separating biometrically the 

 bulk of b from c; on the other hand the depression between the bulk of a and 

 the bulk of b is very insignificant. Although the three subgroups of macram- 

 phidiscs are thus rather closely connected both morphologically and biometri- 

 cally, I think the difference between them sufficient for a separate description. 



The micramphidiscs are very various in size, their length ranging from 16- 

 92 ju. They are, however, morphologically all very much alike. The length 

 frequency-curve pertaining to them is, as the figure shows, exceedingly irregular 

 and has no less than six low elevations in its left part (which pertains to the 

 smaller forms) and one high elevation in its right end-part (which pertains 

 to the largest forms). According to this one might divide these amphidiscs into 

 two subgroups: — large micramphidiscs for those to which the simple high ele- 

 vation in the right-hand end-part of the curve pertains, and small micram- 

 phidiscs for those to which the irregular left-hand and central parts of the 

 curve pertain. Since, however, there is but very little morphological difference 

 between the former and the latter, I refrain from doing so. 



I accordingly distinguish four kinds of amphidiscs in this species: — 1, 

 geminate, 2, large, and 3, small macramphidiscs; and 4, micramphidiscs. 



The geminate macramphidiscs (Plate 104, figs. 1-5, 8, 11, 12; Plate 105, 

 figs. 1-14) are 210-360 m long, most frequently about 278 /x. The shaft is 21-31 m 

 thick, straight or slightly curved, and usually quite smooth and simply cylindri- 

 cal. Sometimes a large, terminally rounded spine or branch, 20-30 /i long, arises 

 from the middle-part of the shaft (Plate 105, fig. 3). When such a spine is present 



