Journal of Paleontology, vol. 19, no. 6, pp. 625-632, pls. 85-87, 1 text fig., November 1945 



AMPLEXOID CORALS FROM THE CHESTER OF ILLINOIS 



AND ARKANSAS 



WM. H. EASTON 



Abstract — Three new corals are described from the Paint Creek, Golconda, and 

 Fayetteville formations, and one from the Renault is redescribed. The ontogenies 

 of three species are discussed and figured in detail. Possible phylogenetic relation- 

 ships are considered. Protosepta are inserted in this order: axial septum, left alar 

 septum, right alar septum, left counter-lateral septum, and right counter-lateral 

 septum. 



INTRODUCTION 



This study is offered to bring to light 

 three new species of Chester corals 

 which can be used in correlation and to pre- 

 sent detailed discussions of the ontogenetic 

 development of Chester species generally 

 referred to Amplexus. A large number of 

 specimens was studied, many of which pos- 

 sessed complete apical portions. 



septal insertion and phylogeny 



Two recent papers contain excellent bib- 

 liographies of ontogenetic studies of rugose 

 corals. Grove (1934) has presented a detailed 

 objective review of the principal studies of 

 fossil corals and Hill (1935, pp. 505, 506) has 

 published a very concise statement of the 

 septal arrangement among the rugose corals, 

 which is quoted below. 



Insertion of meta-septa takes place at four 

 points, 1 immediaately on each side of the cardi- 

 nal septum, and on the counter side of each alar 

 septum. 



Referring, first to the cyathaxonid type, inser- 

 tion of meta-septa is always accompanied by in- 

 sertion of minor septa as follows [fig. lb]: — 2 On 

 each side of the cardinal [C] and both counter-la- 

 teral [CL] septa a new short septum arises. Those 

 [IX] on the counter side of the counter-lateral 

 septa remain short and are minor septa; the oth- 

 ers [1] lengthen, and immediately on each side of 

 them a new short septum arises. That on the 

 counter side [2X] remains short and is a minor 

 septum; that on the cardinal side [2] lengthens to 

 become a major septum, and two new short septa 

 are inserted one on each side of this new major 

 septum, the one on the counter side [3X] being a 

 minor septum, and the other [3] growing to be- 

 come a major septum, and so on. 



1 Exceptions are known. 



2 This and subsequent brackets are the writ- 

 er's. 



In a second group [fig. la, the zaphrentid type], 

 by far the largest, at first after the insertion of the 

 six proto-septa, only meta-septa arise at the four 

 points of insertion, and minor septa (which in 

 this group are obviously delayed) later appear in 

 rapid succession in all the loculi, after which ma- 

 jor and minor septa arise at each point of inser- 

 tion as in the first group. 



It is not yet fully agreed as to whether the 

 rugose corals have four or six primar}' 

 septa, the counter-lateral septa being called 

 metasepta by some students. Disregarding 

 the philosophical implications, however, it 

 has been demonstrated that the order of 

 insertion cf the first six septa is as reviewed 

 by Grove (1934, pp. 119, 120): 



I. A single septum stretches entirely across the 

 calicle, from wall to wall. At a considerably later 

 stage this breaks near the center, the two halves 

 forming the cardinal and counter septa. This 

 first formed septum Carruthers designated as 

 the "axial septum." 



II. Two new septa arise, one on either side of 

 the axial septum, at its cardinal end. These re- 

 main attached to the wall of the corallum and to 

 the axial septum, but gradually spread outwards, 

 and eventually become the alar septa. 



III. A second pair of septa appear at the 

 counter end of the axial septum, and similarly 

 spread outward, but to a much more limited ex- 

 tent than the first pair. 



Grove goes on to say that the "disruption 

 of the axial septum completes the formation 

 of six primary septa," but this does not 

 always happen at this stage; indeed, it may 

 not happen until a much later stage, if at all. 



All four of the amplexoid corals considered 

 here have the zaphrentid type of septal in- 

 sertion. The species restricted to the Paint 

 Creek formation differs from the others in 

 having an early mature stage in which the 

 septa of the counter quadrants are dilated 

 and in having short or rudimentary minor 



625 



