Protiiitplanktoii. 



pitM'ces ill a direction obliquely ilownwanls. In this way it un- 

 doubtedly takes an iuipoitant part in the foiniation of the thonix. 

 partly with brandies, partly witli arches to the other primary spines. 

 The spine I) runs nearly in the cephalis wall, oblitjuely upwards, 

 as an apical horn - .,topliorn" (IIok.) — which is but little con- 

 spicuous, while the spines l^^. iinil l.| pierce tlie tlmnt.r in, or a 



little below, tlie ..neck". Tlie ventral sa-iltai spine protrudes 

 obliiiuely from the region of the neck as a lower ..frontal horn" 

 (HcK., in contradistinction to the „apical spine" as a ..hitrher 

 occipital horn"). 



The peculiar swellings seen in tlie reL;ion of the neck in 

 Lithomdifsa getosa and similar forms, will tind a natural explaiiatiuu 

 throutrh the above mentioned process of formation of the thorax, 

 and this also gives an explanation of the indistinct outer boundary 

 between the ct'iihulis and the thorax. The i)roper liouiidary is to 

 be found near the primary arches, where it is dclined liy internal 

 lists („cortinar septum" as IIck. mentions in some species). These 

 are not in the same plane, just as in Phrtacaiitha and Perklium. 

 As the secondary spines on the left and right lateral arch through 

 additional secondary arches now form the adjoining part of the 

 thorax, two crimpings on the sides about the spine D will be 

 caused at the boundary between the cephalis and thorax. 



In Cyrtoidea there is also developed an inner spine, which 

 extends from the central rod through the thorax. It appears to 

 be very variously developed, and is sometimes branched. It always 

 seems to extend from the ventral end of the central rod, at the 

 -same point which is also the starting point for the primary, lateral 

 spines and the A'entral sagittal one, and it extends in a direction 

 contrary to the cephalis. Its object seems to be to protect the 

 large opening, which is the weak point, or perhaps it is to act as 

 a support for the lobes of the central capsule. As a rudimentary 

 spine, it can be traced right back to Phorjiiacantha. 



The more richly developed the Cijrtoidea forms become, the 

 more it seems that the original spines and arches are retrograded. 

 The four primary spines, as well as the ventral, sagittal spine can, 

 however, be clearly recognized in every instance, as far as my 

 experience goes. In the genera Helotholus, Androcyclas and Clathro- 

 cijclas, perhaps also Sticlwcori/s, 3 of these spines, A, Lj. and Li, 



are found as simple, protruding spikes. In the genus Androcyclas 

 the spine D forms the large, substantial „to])horn" (llcic). Sim- 

 ilarly in Clathrocyclas. where the other ..tophorn" is formed by the 

 protruding ventral sagittal spine. 



The consecutive links of the many linked Cyrtoidea are formed 

 out from the cephalis as may easily be proved in young individuals. 

 Such forms as Lithomelissa will, therefore, when young resemble 1 

 Peridium, Clathrocycla." and Androcyclas and be similar to Dicyrtida 

 (instead of Tricyrtidn) etc. 



The genus Amphimelissa (Botryopylc si'tosa ('i..) seems to 

 have a structure which is considerably ditferent to that of Litho- 1 

 melissa, with a fuller development of the primary skeleton parts 

 of the cephalis. If I have understood it rightly, it would seem | 

 that its thorax is formed from the cephalis by secondary spines on 

 the secondary arches, that is with the starting point higher up on 

 the cephalis, while the thorax of Lithomelissa and similar genera 

 are chiefly formed from the i)rimary arches and their byspines. The i 

 result is therefore, that Amphimelissa has a broader cephalis, which [ 

 is more enclo.sed, or even entirely so, and which does not distinctly 

 appear to be separated from the thorax. (Cfr. further under Am- | 

 2)hivtelissaJ. 



The genealoLncal tree for the forms which 

 will, after the foregoing, be as follows: 



Sliclwcorys 



ccur in my mateiial 



Dictyoceras 



Amphimelissa 



\ 



Dirtyophvnins 

 Androcyclas 



Helotholus 



Litho 



Pteroscenua 



Cladosceni/ 



Phormacantha 



Ceraiospyris 



\ 



Peridium Dicti/ocircus 



/ / 



Plcctucantka / 



Camp])lacantha 



iToundform with 4 primary spines 



In the following paires. I have for practical reasons preferred 

 to retain (at any rate preliminarily) Haeckels method of placing 

 the Cyrtoidea, where therefore the ..apical spine" is the spine D, 

 and the „dorsaI one" the spine A, while further the right and 

 left lateral spines change places. Where, however, letters are 

 used, I have applied them as above. 



W"\{\\ regard to the distribution of the species of Xasstlhiri<( 

 which 1 liave observed, I will add a few remarks, and these will 

 also, as a whole, be applicable to the above mentioned species of 

 the division Spumellaria. 



It is difficult, from the material treated, to come to any re- 

 liable conclusion as to whence the various species really come, and 

 this indeed is the case with most of the plankton species which 

 occur in my material. \\'c know very little indeed about the di- 

 stribution of these radiotaria. for the majority of the species found 

 are cither quite new or had only previously been known from the 

 west coast of Norway. But, fortunately, Cleve has also observed 

 some of the species in question (especially from the sea near Spitz- 

 bergen), so that the distribution of just these species can be rather 

 more completely stated. It has also been of great service to me 

 that Dr. ILjout and Dr. Ghax have kindly favoured me with a number 

 of plankton samples for examination which were taken on S/S Mi- 

 chael Sars' expedition in the Norwegian Ocean. I have, however, 

 not yet been able to find time to work through the whole of this 

 material, which, having been collected from settled places in the 

 most inq)ortant currents, will supi)Iy valuable information with i 



