HYALONEMA (OONEMA) DENSUM. 
347 
cally by a wide gap in the length frequency-curve only from group A. Groups 
C and D differ morphologically greatly from A and B but not nearly so much 
from each other. Biometrically, that is judging from the width of the depres- 
sion separating the two elevations of the curve pertaining to them, they are 
also less clearly distinguished from each other than A is from B. 
Thus C and D together form a main group, which is to be named micramphi- 
discs, as it comprises the smallest amphidiscs. Although separated by a wide 
gap in the curve, and differing also morphologically, I am inclined to combine 
A and B in a like manner in one main group, which is to be named macramphi- 
discs, as it comprises the largest amphidiscs. Within each of these main groups 
I distinguish two subgroups differing in size, and thus divide the amphidiscs 
into the four groups: — large macramphidiscs (morphological group A); small 
macramphidiscs (morphological group B ); large micramphidiscs (morphologi- 
cal group C ) ; and small micramphidiscs (morphological group D). 
The large macramphidiscs (Plate 96 , figs. 8, 9, 14) are rare. They are 
450-540 fi long, most frequently about 476 /x, and have a straight shaft, 21- 
29 n thick. The shaft is either quite simple and cylindrical throughout (Plate 
96 , fig. 9), or it bears a rounded protuberance or two, about 10 n high, in its 
middle part (Plate 96 , figs. 8, 14). The terminal anchors are 125-140 n long, 
less than a third of the whole spicule, and 190-230 n broad. The proportion of 
length to breadth of the anchors is 100 to 145-174, on an average 100 : 162. 
The anchors are composed of eight teeth. The individual teeth are throughout 
curved fairly uniformly and sharply pointed at the end. 
The small macramphidiscs (Plate 96 , figs. 1-7, 10-13) are present in fair 
numbers, but are not nearly so abundant as in the allied species. They are 90- 
184 n long, most frequently about 135.6 /x. The shaft is straight, simply cylin- 
drical, and 9-15 n thick. The terminal anchors are 45-92 n long, usually a little 
more than half the whole spicule. Their breadth is 60-136 /x. The proportion 
of anchor-length to anchor-breadth is 100 to 125-157, on an average 100 : 146.2. 
The anchors are usually composed of eight, more rarely of seven teeth. The 
teeth of the two anchors of the same spicule are usually situated so that those 
of the one anchor alternate regularly with those of the other. The individual 
teeth consist of an outer band-shaped part, up to 30 /x broad, and simply rounded 
at the end, and an inner keel, high at the base and uniformly narrowing dis- 
tally. The outer contour is more strongly curved in its proximal and distal 
than in its middle-parts. At the end of the tooth it is always strongly bent 
inwards. 
