Remarks:Spongosaturninus Campbell and Clark is regarded as a junior synonym of Acanthocircus Squinabol. Spongosaturninus like Acanthocircus shows a latticed medullary shell(s) and a spongy cortical shell. Spongosaturnalis Campbell and Clark is probably a junior synonym of Acanthocircus. The type species seems to possess a small latticed medullary shell. It is likely that forms with auxillary spines [e.g. A. (?) minensis (Yao)] will have to be placed in a separate genus.
Stratigraphic range:Triassic to Cretaceous so far as is known.
Subsequent descriptions:YES

Emended description:Formen mit durch Leisten deutlich gegliedertem Ring, der nicht durchgehend bestachelt ist. Die Bestachelung ist meist ± senkrecht zu den Polarstäben ausgerichtet, z.T. treten auch in derselben Position Dornen auf. Bedornung in Kombination mit Bestachelung kann auch den Nahbereich der Polarstäbe erfassen (d.h. mehr oder minder parallel zu den Polarstäben verlaufen) . Außer den beiden Polarstäben sind keine Hilfs- und Stützstäbe entwickelt. Rinden- und Markschalen siehe Familiendiagnose.
[Forms with a ring clearly divided by a transverse bar with no transverse spines. Frequently, spines more or less perpendicular to polar ends, sturdy thorns being also located in same areas. Thorns combined with spines can be located near a polar end of the shell (i.e . more or less parallel to the polar area). Nor secondary or additional bars developed on parts of the ring other than the polar areas. Cortical and medullary shells as in family.]
Subsequent descriptions:YES

Emended description:Single unlatticed ring with two opposed polar spines; ring with peripheral spines projecting outward. Polar spines with fragmentary thorns (Ft) and sturdy spines (Ss) (pl. 6, figs. 12-14). Fragmentary thorns marking points of attachment of concentrically layered spongy cortical shell; sturdy spines marking attachment of latticed medullary shells.
Remarks:Acanthocircus differs from Parasaturnalis and Japonisaturnalis in having a single rather than a double ring; furthermore, Japonisaturnalis has a variable number of polar spines. Acanthocircus differs from Spongosaturnaloides in having 2 rather than 5 polar spines and from Austrisaturnalis in having a solid ring and 2 rather than 4 polar spines. Acanthocircus differs from Pseudosaturnalis in having a solid rather than a latticed ring. Spongosaturnalis Campbell and Clark and Spongosaturninus Campbell and Clark are regarded as junior synonyms of Acanthocircus. The type species of both genera show a spongy cortical shell and latticed medullary shells. Kozur and Mostler (1972, p. 30) regarded Acanthocircus as a synonym of Spongosaturnalis but insisted on using Campbell and Clark's junior name in their report.
It should be noted that the form figured by Pessagno (1977a, pl. 3. figs. 7-12) as Acanthocircus sp.A seems ancestral to the type species of Acanthocircus. The electron micrographs clearly demonstrate the presence of a spongy cortical shell and latticed medullary shells.
It now appears that the fauna described by Squinabol (1903b) is Albian or Cenomanian in age. All ringed Spumellariina with 2 polar spines examined from strata of this age possess typical polar spines with fragmentary spines and sturdy spines (after Yao, 1972). Even if the spongy cortical shell and latticed medullary shells have been broken away, their presence can be documented by a careful analysis of polar spine structure.
The following species erected by Kozur and Mostler (1972) and assigned to Spongosaturnalis should be reassigned to Acanthocircus: S. rotundus, S. elegans. S. fluegeli, S. fissus, S. brevispinosus, and S. triassicus.
Stratigraphic range:Triassic. Cretaceous. Mesozoic.
Subsequent descriptions:YES

Remarks about the type species:[Kozur and Mostler give as type species: Saturnulus trizonalis Rüst 1898 = ? Acanthocircus irregularis Squinabol, 1903]
Revised description:Unit strongly elongated perpendicularly in the direction of the axis of the polar spines by a ring which is mostly strongly elongated in this direction, by long terminal spines on the poles perpendicularly to the polar spines or by both features together. Ring very narrow, bladed or with swellings or furrows, only in the most primitive taxa not differentiated, but with round or oval cross section. Outer margin of ring rarely smooth, mostly with spines in the polar region of the long axis, sometimes with spines along its outer margin. Two polar spines. No other first order spines or auxiliary spines. Cortical shell spongy, medullary shell tiny, latticed.
Remarks:Spongosaturnalis Campbell and Clark, 1944b, always has a totally spined ring of circular to subcircular outline. Palaeosaturnalis Donofrio and Mostler, 1978, is distinguished by a circular, always flat and undifferentiated, mostly broad to very broad spiny ring. Additional to the 2 polar spines Praeacanthocircus n.gen. still has 4 spines which have almost the same size as the polar spines.
Stratigraphic range:Lias-Late Cretaceous. Mesozoic.
Subsequent descriptions:YES

Literature cited:- Campbell, A. S. and B. L. Clark (1944b). Radiolaria from Upper Cretaceous of Middle California. Geological Society of America Special Paper 57: 1-61.
- Donofrio, D. and H. Mostler (1978). Zur Verbreitung der Saturnalidae (Radiolaria) im Mesozoikum der Nördlichen Kalkalpan und Südalpen. Geologisch Paläontlogische Mitteilungen Innsbruck 7(5): 1-55.
- Kozur, H. and H. Mostler (1972). Beiträge zur Erforschung der mesozoischen Radiolarien. Teil I: Revision der Überfamilie Coccodiscacea Haeckel 1862 emend. und Beschreibung ihrer triassischen Vertreter. Geologisch Ð Paläontologische Mitteilungen Innsbruck 2: 60.
- Pessagno, E. A. (1977a). Radiolaria in Mesozoic stratigraphy. Oceanic Micropalaeontology. A. T. S. Ramsay. London/New York/San Francisco, Academic Press. 2: 913-950.
- Pessagno, E. A., W. Finch, and P.L. Abbott (1979). Upper Triassic Radiolaria from the San Hipolito Formation, Baja California. Micropaleontology 25(2): 160-197.
- Rüst, D. (1898). Neue Beiträge zur Kenntniss der Fossilen Radiolarien aus Gesteinen des Jura und der Kreide. Paläontographica 45: 1-67.
- Squinabol, S. (1903b). Le Radiolarie dei noduli selciosi nella Scaglia degli Euganei. Contribuzione I. Rivista Italiana di Paleontologia 9: 105-151.
- Yao, A. (1972). Radiolarian fauna from the Mino Belt in the northern part of the Inuyama Area, Central Japan, Part I: Spongosaturnalids. Journal of Geosciences, Osaka City University 15: 21-65.