REPORT ON THE RADIOLARIA. 725 



exoplasm on the surface of the calymma. These and other differentiations seem to 

 indicate that the pseudopodia in the Acantharia are more highly developed than in 

 the Spqmellaria, and justify the denomination of the former as " Actipylea." 



Synoiisis of the Orders and Suborders q/" Acantharia. 



I. ACANTHOMETEA. 



Skeleton composed only of 

 acanthinic radial spines not 

 forming a complete lattice- 

 shell. 



II. ACANTHOPHEACTA. 



Skeleton composed of twenty 

 acantbmic radial spines (dis- 

 posed after the Miillerian law) 

 and of a spherical or variously 

 shaped complete lattice-shell. 



Puidial spines in variable and indefinite number, 



disposed irregularly, . . . .1. Actinelida. 



Radial spines constantly twenty, disposed regularly 



after the Miillerian law of fcosacantha, . 2. A c a n t h o n i d a. 



Eadial spines all twenty of equal size ; shell and 



central capsule spherical, . . . 3. Sphserophracta. 



Radial spines of diflferent sizes ; shell and central 



capsule ellipsoidal, discoidal, or heteromorphous, 4. Prunophracta. 



Order III. ACANTHOMETEA, Johannes Mliller, 1855. 



Acanthor)ietra, J. Midler, 1855, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin. 

 Acanthometi'ida, Haeckel, 1862, Monogr. d. Eadiol., p. 371. 

 Acanthometrea, E. Hertwig, 1879, Organismus d. Eadiol., p. 133. 

 Acanthonida et Litholopkida, Haeckel, 1881, Prodromus, pp. 465, 469. 



Definition. — Acantharia without complete latticed shell. 



The order A c a n t h o m e t r a, the thiixl order of Eadiolaria, comprises all those 

 Acantharia in which the acanthinic skeleton is only composed of radial spines arising 

 from one common central point, but never forms a complete latticed shell. By the 

 aljsence of such a latticed or fenestrated shell the Acanthometra diifer principally 

 from the nearly allied Acanthophracta, the second order of Acantharia, which 

 constantly possess such a complete shell. 



Johannes Midler, who first detected and described the Acanthometra (in 

 1855, loc. cit.), defined them as follows: — "Eadiolaria without shell, with siliceous 

 radial spines" (1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 46). He described and 

 figured eighteen species of them, disposed in four genera {Acanthometra with fifteen 

 species, and Zygacanfha, Lithophyllium, Lithoj^tera, each with a single species). 

 Among those eighteen species, however, were two " Acanthometrse cataphract^," apj)er- 

 taining to the following order, the A c a n t h o jj h r a c t a. 



In my Monograph (1862, p. 371) all true Acanthometra were united into a 

 single family, Acauthometrida, with the following definition : — " Skeleton composed of a 

 number of radial spines, piercing the central capsule and united in its centre, without 



