AMP LEX ID CORALS FROM ILLINOIS AND ARKANSAS 631 



4.5 cm.; average diameter at calyx about 

 9 mm. 



Transverse sections. — -Septa thin, relative- 

 ly lew in number, generally meet at axis; 

 tabular intersections common, straight. (See 

 "Ontogeny" for details.) 



Longitudinal section. — Tabulae about 4 

 mm. apart, highly arched distally, the pe- 

 ripheral edges sloping proximally from 

 counter to cardinal position. 



Occurrence. — The type locality was origi- 

 nally given as "Chester limestone, Pope 

 County, Illinois," but subsequent studies 

 have shown the species to be restricted to 

 the Shetlerville member of the Renault lime- 

 stone of Lower Chester age. 



Material. — Specimens studied, 62; holo- 

 type, Illinois State Geological Survey 

 (Worthen collection) No. 2566a; paratype, 

 same collection, No. 2566b; figured plesio- 

 types, Illinois State Geological Survey No. 

 3526-3531, 3536, and University of Illinois 

 No. X-1479; unfigured plesiotypes, Illinois 

 State Geological Survey Nos. 3532-3535. 

 The writer has studied Illinois specimens 

 from the grey-green shale cropping out on 

 the east bank of the ravine above the old 

 Bean and Mc Master's quarry near Shetler- 

 ville; from f mile south of Eichorn; from 

 Melcher Hill in Hardin County; and from 

 the ravine east of Shetlerville. 



Ontogeny. — Before any septa are inserted 

 (pi. 87, fig. 8a) there is simply a ring of epi- 

 theca. Presumably septa are inserted in an 

 orderly manner as in the other species 

 studied, in which case the axial septum ex- 

 tended across the thecarium, then the left 

 alar, right alar and left counter-lateral septa 

 were inserted (fig. 8b) but these stages have 

 not been observed. The alar septa migrate 



toward the counter septum until they stand 

 at right angles with the axial septum, by 

 which time the right counter-lateral septum 

 has appeared (fig. 8c). The next observed 

 stage (fig. 8d) shows one metaseptum in 

 each cardinal quadrant and one metaseptum 

 and the counter-lateral septum in the left 

 counter quadrant and the counter-lateral in 

 the right counter quadrant. These relation- 

 ships persist a short time (through fig. 8e). 

 but in the next section (fig. 10) each quadrant 

 has one metaseptum in addition to a 

 counter-lateral septum, and tabulae occur. 

 Subsequently, acceleration of the counter 

 quadrants occurs and they contain four or 

 five septa while the cardinal quadrants con- 

 tain two or three metasepta (fig. 11). By 

 this stage most of the metasepta are with- 

 drawn from the axis, although they have 

 shown an impersistent tendency to retreat 

 since an early stage (fig. 8c). Moreover, the 

 epitheca is lined with septal grooves between 

 major septa, but this suggestion of minor 

 septa is probably misleading because the 

 grooves seem to correspond with longitudi- 

 nal wrinkles inside the thecarium, rather 

 than to actual rudimentary minor septa. 

 Later, rudimentary minor septa are defi- 

 nitely present. At full maturity, the septa 

 are usually about 20 to 24 in number and 

 occur in many different patterns, varying 

 between a radial arrangement in which most 

 septa nearly or quite reach the axis (fig. 14) 

 and the typical amplexoid pattern (fig. 12) 

 in which the septa are withdrawn toward the 

 periphery and do not extend vertically 

 throughout their length between adjacent 

 tabulae. An intermediate pattern (fig. 13) is 

 obtained if the plane of a section passes just 

 below a tabula. 



Explanation of Plate 87 



Figs. 1, 6, 13 — Amplexus adnatus Easton, n. sp.; paratypes; Walker Museum collections; X5. la-f — 

 7, Transverse serial sections; reversed; 1 specimen. 2-4 — Transverse sections; reversed; 3 

 specimens. 5-13 — Transverse sections; 2 specimens. 6 — Longitudinal section. 7 — Trans- 

 verse section just below a tabula. (p. 627) 

 8-12, 14 — Amplexus geniculatus Worthen; plesiotypes; Illinois State Geological Survey collec- 

 tion, except fig. 13; X5. 8a-e — Transverse serial sections; reversed; 1 individual; No. 3526. 

 9 — Longitudinal section in cardinal-counter plane; No. 3527. 10-12 — Transverse section; 

 reversed; 3 specimens; Nos. 3528, 3529, 3530. 13 — Transverse section; No. 3526. 14 — Trans- 

 verse section, reversed; University of Illinois No. X-1479. (p. 630) 



