AMPLEXOID CORALS FROM ILLINOIS AND ARKANSAS 627 



in normal sequence, are not observable until 

 after a counter-lateral septum appears, their 

 actual presence apparently being indicated, 

 however, by slight internal swelling of the 

 epitheca (pi. 86, fig. 2a). Further study of 

 sections in the same sequence bears out the 

 observation that the arrangement of septa is 

 independent of the effects of attachment, 

 for the plane of attachment cuts across the 

 plan of septal arrangement without their 

 being any adjustment of the center of sym- 

 metry. 



ACKNOWLEDGMENTS 



The writer is indebted to Carey Croneis 

 of Walker Museum, University of Chicago, 

 and R. C. Moore of the University of 

 Kansas, for lending specimens used in this 

 study. Some locality data were furnished by 

 J. Marvin Weller. 



systematic descriptions 



Phylum Coelenterata 



Class Anthozoa 



Order Tetracoralla 



Family Hapsiphyllidae 



Genus Amplexus of authors 



Diagnosis. — Simple, conico-cylindrical, 

 rugose corals with broad generally complete 

 tabulae; septa more or less radial in arrange- 

 ment, extending to the axis over the upper 

 surfaces of most tabulae, but extending 

 distally from each tabula a progressively 

 shorter distance as the axis is approached; 

 dissepiments and columellar structure ab- 

 sent. 



Occurrence. — Silurian to Permian of many 

 parts of the world. 



Remarks. — Amplexus (sensu stricto) was 

 founded upon A. coralloides by Sowerby in 

 1814, but since that time a wide variety of 

 species has been referred to this genus on the 

 basis of their simple structure. Type ma- 

 terial of A. coralloides has never been sec- 

 tioned in early stages, hence, the morphol- 

 ogy of the genotype is inadequately known. 

 Paleontologists now seem to be generally 

 agreed that corals of simple structure similar 

 to A. coralloides may very well belong to 

 some genus other than Amplexus, but onto- 

 gentic studies still largely remain to be done 

 before polyphyletic relationships of 'Am- 

 plexus" can be demonstrated satisfactorily. 



. Amplexus adnatus Easton, n. sp. 

 Plate 85, figure 9; plate 87, figures 1-7 



Externals. — Ceratoid to trochoid genicu- 

 late corals with apical angle usually about 

 2Q°; epitheca with numerous rather fine 

 encircling wrinkles, prominent septal grooves 

 and interseptal ridges which always origi- 

 nate in connection with a pair of adjacent 

 septa; spines commonly extending from one 

 side, especially near apical end, parracidal 

 increase common, with new calyx starting 

 from some eccentric position and with 

 epitheca commonly grown over old calyx. 

 Average length about 20 mm. with range 

 from about 10 to 30 mm.; average diameter 

 about 6 mm., but flared specimens may be 

 11 mm. in diameter at calyces. 



Transverse sections. — Septa not numerous, 

 mostly reaching center, even in late stages. 

 (See "ontogeny" for details.) 



Longitudinal section. — Tabulae nearly all 

 complete, with little or no axial sag, rela- 

 tively abruptly recurved proximally near 

 peripheries. 



Occurrence. — Abundant in the Golconda 

 formation in the Vienna quadrangle. Col- 

 lected by Stuart Weller from his locality W- 

 312, which may be the E±, NW£, sec. 28, 

 T. 13 S., R. 3 E., near top of the bluff on the 

 high point of land between two creeks A\ 

 miles south-southeast of Vienna, Illinois; 

 also occurring in the Paint Creek formation. 



Material. — Holotype, No. 47248; para- 

 types and topotypes, not numbered; all in 

 Walker Museum, University of Chicago. 

 Ideotypes, Walker Museum No. 47249, 

 Illinois State Geological Survey No. 3537. 



Ontogeny. — The earliest stage observed 

 (pi. 87, fig. la) has an axial septum which 

 consists of the fused cardinal septum and 

 counter septum. In the next observed stage 

 (fig. lb) both alar septa are present, of which 

 the left is the thicker and has migrated far- 

 ther from the cardinal septum, therefore, it 

 probably was inserted before the right alar. 

 Other stages were not observed before that 

 in which the left cardinal quadrant contains 

 one metaseptum, the right cardinal quadrant 

 none, the left counter quadrant the probable 

 counter-lateral septum, and the right coun- 

 ter quadrant one metaseptum and the 

 counter-lateral septum (fig. lc). This asym- 

 metrical arrangement presumably is a mani- 

 festation of geniculation of the corallite. 



