630 



WM. H. EASTON 



in the very early unflared portion of the 

 corallites (pi. 86, figs, la-lh, 2a-2h). Al- 

 though not actually observed, the earliest 

 stage presumably is aseptate. The earliest 

 observed stage (fig. la) has an axial septum 

 extending across the center of the thecarium 

 consisting of the fused cardinal septum and 

 counter septum. In every observed instance, 

 the cardinal septum is more or less normal 

 to the plane of attachment of the corallite. 

 At the next stage (fig. 2a) the left counter- 

 lateral septum is inserted but this apparent- 

 ly unorthodox sequence is caused by the alar 

 septa being crowded against the epitheca 

 parallel with the plane of attachment and 

 only faintly observable in sections; they are 

 first clearly seen at the next stage (fig. 2b) 

 which has an axial septum, counter-lateral 

 septa, and the fully developed alar septa 

 starting to become separate from the epi- 

 theca. The coral grows larger (fig. 2c) and 

 eventually adds a metaseptum in the left 

 counter quadrant (fig. 2d) and then another 

 in the right counter quadrant (fig. 2e). This 

 corresponds to about the same stage shown 

 in figure lb, but the latter specimen is aber- 

 rant in having the center of symmetry offset 

 toward the counter position. There may be 

 a tendency after this stage for the septa to 

 retreat from the axis (figs, lc— If), but even 

 though this occurs, the septa may reverse 

 the trend (figs, lg-li) before beginning their 

 final retreat (fig. lj) and attaining maturity 

 (fig. 2i). After two metasepta have been in- 

 serted along with the alar septa, a meta- 

 septum appears in the left cardinal quadrant 

 (fig. lc) and then another in the right cardi- 

 nal quadrant (fig. 2f). When each counter 

 quadrant has four septa, and the cardinal 

 quadrants each have two metasepta, the 

 final septal retreat (fig. li) starts by axial 

 thinning of septa. Alar septa and the counter 

 septum are longer than other major septa 



and the cardinal septum soon becomes quite 

 short (fig. lj), but in maturity (fig. 2i) 

 the alar septa and the cardinal septum are 

 shorter than the others and the counter 

 septum is the same length as the neighboring 

 metasepta. 



Comparison. — This species is readily iden- 

 tified externally by the flaring corallites and 

 by the brephic stage in which the epitheca 

 is equipped with very fine encircling striae 

 and bears the flattened area where it was 

 attached to some foreign object (patterns of 

 costate brachiopod shells were commonly 

 observed). These features serve to distin- 

 guish A. expansus from other Chester 

 amplexoid corals. 



Remarks. — Ontogenetically, this species is 

 characterized by early and rapid retreat of 

 the major septa, by insertion of about half 

 the total number of major septa before 

 notable flaring of the calyx begins, by nearly 

 radial arrangement of septa in late stages, 

 and by nearly uniform thickness of major 

 septa in late stages. 



Phylogenetic relationship is with the so- 

 called "zaphrentid" types but the species 

 is clearly of different ancestry than the other 

 Chester "Amplexus." 



Amplexus geniculatus Worthen 



Plate 85, figure 7; plate 87, figures 8-14 



Amplexus geniculatus Worthen, 1890, Illinois 



Geol. Survey, vol. 8, p. 82, pi. 10, figs. 7, 7a. 



Externals. — Large simple corals, common- 

 ly geniculate near apical end and oc- 

 casionally contorted in cylindrical portion; 

 calyx rather deep, nearly vertically walled; 

 epitheca occasionally spinose but always 

 with very prominent septal grooves and 

 interseptal ridges; corallites commonly cera- 

 toid to trochoid near apical end, with abrupt 

 change to nearly cylindrical shape, or 

 gradually expanding. Average length about 



Explanation of Plate 86 



Figs. 1-3 — Amplexus expansus Easton, n. sp.; paratypes; University of Kansas collections; X5. 



la-j — Transverse serial sections; reversed; 1 specimen. 2a-i — Transverse serial sections; 



reversed; 1 specimen. 3 — Longitudinal section. (p. 629) 



4-5 — Amplexus dilatatus Easton, n. sp.; Walker Museum collections; X5. 4a-b — Transverse 



sections; holotype; No. 47247. 5 — Transverse section; reversed; paratype. (p. 628) 



