290 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The elongation of the cirri, which is accompanied by an increase in stoutness, 

 is in its true phylogenetic aspect also almost exclusively peculiar to the Oligo- 

 phreata, occurring in the Macrophreata only in the single genus Zenometra (figs. 109, 

 p. 175, and 377, 378, p. 301). It is best studied in the Thalassometridse andChari- 

 tometridEe. In the Charitometridse (figs. 99, p. 160, 100, p. 162, and 369, 370, p. 299) 

 the cirri are short and stout, the segments, except for the first two, subequal, 

 usually somewhat longer than broad, becoming slightly shorter distally and smooth 

 dorsally, though the distal dorsal ends of the outer segments may be somewhat 

 swollen. Their surface is dull, due to the presence of a close fine pitting, and 

 their general coloration is dark, like that of the calyx and arm bases. The 

 terminal claw and the distal margin of the penultimate segment, however, have a 

 highly polished surface and are comparatively light in color. 



In the Thalassometridse (figs. 93, p. 153, 94, p. 155, 95, p. 157, 96, 97, p. 159, 361, 

 362, p. 295, and 363-368, p. 297) the proximal cirrus segments for a variable dis- 

 tance from the centrodorsal are well rounded in cross section, smooth, stout, and 

 comparatively dark in color, resembling exactly those of the Charitometridse ; then 

 comes a peculiar segment which I have designated as a transition segment (fig. 4, 

 p. 63). This transition segment typically decreases more or less in dorsoventral 

 diameter distally, and rather more rapidly in transverse diameter. In its proximal 

 half to three-fourths it is dark in color and in every way resembles the preceding 

 segments, but in its distal fourth to half it is highly polished and more or less later- 

 ally compressed, and light in color, and it bears a median projection on the distal 

 dorsal edge. Usually this segment is especially marked by a dark band about it 

 at the dividing line between the dull proximal and polished distal portions. 



In its structure, and in its position in reference to the segments comparable 

 morphologically to its proximal portion (the preceding segments), it is the homo- 

 logue of the penultimate segment as seen in the Charitometridse; but instead of 

 bearing a terminal spine it is succeeded by a series composed of a variable number 

 of short spinous highly polished segments which eventually terminate in a penulti- 

 mate segment and terminal claw as usual. 



Considering the transition segment as representing the penultimate segment 

 of the Charitometridse, the cirri of the Charitometridse as a whole are the equivalent 

 of that part of the cirri of the Thalassometridae up to and including the transition 

 segment. The segments found in the cirri of the Thalassometridse beyond the 

 transition segment I interpret as additional segments morphologically the result 

 of budding or of a process of progressive serial reduplication from the primitive 

 penultimate segment as seen in the Charitometridse, as a result of a phylogenetically 

 sudden increase in the length of the cirri over the short charitometrid type. The 

 typical elongation of the cirri as found in the Oligophreata, therefore, is not the 

 result of a phylogenetically gradual increase in the number of cirrus segments as in 

 the Macrophreata, but of a process of phylogenetically abrupt and sudden distal 

 elongation. 



In the Thalassometridse this transition segment is especially marked, and it 

 is almost equally evident in certain species of the Zygometridse, Mariametridas, 

 Comasteridse, and of other families; but often it has lost, through the disappearance 



