:526 " THETIS " SCIENTIFIC RESULTS. 



contains ten large plates, five orals and five basals ; no radial plates 

 are visible. ]n a specimen a trifle older the radials have arisen in 

 the space bounded by two oral and two basal plates, and the stem- 

 joints are not so discoidal. At a still later stage (PI. xlvii., fig. 8) 

 the radials are conspicuous, though the orals and basals are equally 

 prominent, the first costal and the costal axillary appear as rather 

 elongated plates, and the arms consist of two or three joints ; the 

 stem, which is relatively far more slender than at the earlier 

 stage, consists of ten-twelve joints, of which the middle ones are 

 longer than wide, and are encircled at the mid-zone by a con- 

 spicuous ridge, a possible indication of a circle of cirri in some 

 ancestral form. A similar ridge occurs on the stem-joints of 

 Metacrinus cingulatus ( and other species of the genus), where it 

 appears to be purely ornamental. The older stages (PL xlvii., 

 fig. 9) have the stem and attaching disc of approximately the same 

 actual size, as earlier, and therefore relatively much smaller ; the 

 stem-joints liave lost the encircling ridge and are practically 

 smooth ; the basals are relatively small, but the orals are still con- 

 spicuous ; the cirri have begun to appear on the margin of the 

 centro-dorsal ; they have a variable number of joints, of course; 

 and are very erect, parallel with the arms ; the latter have 

 elongated and the brachials are well defined ; no syzygies are to 

 be found, but the third and fourth plates appear to be somewhat 

 closer together than the others. In the earliest free-swimming 

 stage which I have found, the arms are about 7 mm. long, and have 

 about nine pinnules on each side ; distinct syzygies are present 

 between brachials three and four, seven and eight, and eleven and 

 twelve, but in the remaining sixteen-eighteen joints syzygies have 

 not yet developed; the first pinnule has only four or five joints, but 

 is somewhat stouter than the succeeding ; there are six cirri, each 

 with about thirty joints. Basal plates can no longer be distin- 

 guished, and the oral plates are reduced to a few large irregular 

 fragments in each interradius of the disc. The transition from 

 this stage to the adult condition is obvious. 



I have placed this interesting species in the genus Himero- 

 metra, only after considerable hesitation on account of its small 

 size, the absence of any swelling or convexity on the costals, and 

 the very small number of cirri. It resembles Oligometra in many 

 respects, but the large number of cirrus-joints distinguishes it 

 from the species of that genus. It has seemed to me better to 

 include it in the large and somewhat heterogenous group, Himero- 

 metra, rather than to modify the definition of a smaller, natural 

 genus like Oligometra. 



It might be thought that we have here simply a young form of 

 Ptilometra, especially as these specimens occur only with the 

 .adults of that genus. Fortunately, however, there are young 



