64 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Subfamily CYCLOCCELIIN^ Schuchert and 

 Cooper 1931 



Plectorthidje which probably originated in the Plect- 

 orthins, and in which the hinge-line and cardinal areas 

 are so narrowed as to produce a rostrate shell sugges- 

 tive of the rhynchonellids. 



The subfamily embraces but the one genus, Cyclo- 

 coella, restricted to the Upper Ordovician of the Ohio 

 Valley. 



Genus CYCLOCCELIA Foerste 1909 



(Encuclodema Foerste 1912)^* 



PI. 10, figs. 1-5, 7, 9 



Foerste, Bull. Sci. Lab. Denison Univ., vol. 14, 1909, p. 

 227; vol. 16, 1910, p. 36, pi. 2, fig. 10, pi. 6, figs. 

 8a, b. 



Genoholotype. — jltryfasordidaHa]!, 1847, Pal. 

 N. Y., vol. 1, p. 148, pi. 33, fig. 16. 



Description. Exterior. — Shell small, rhyncho- 

 nelliform to subcircular, anterior and lateral margins 

 convex, hinge-line very narrow; lateral profile bicon- 

 vex, lateral commissure unflexed; anterior commissure 

 rectimarginate to faintly uniplicate; interarea of the 

 ventral valve narrowly triangular, apsacline; beak 

 subrostrate, delthyrium open, umbo convex; dorsal 

 interarea very short, apsacline, umbo very convex. 

 Ornamentation pauci- to multicostate. Test very thin, 

 impunctate. 



Ventral interior. — Delthyrial cavity moderately 

 deep; teeth strong; dental plates sharply defined, 

 strong, diverging, extending nearly one-fourth the 

 length of the shell; muscle area not impressed on the 

 thin shell. 



Dorsal interior. — Cardinalia confined, like those of 

 Plectorthis ; brachiophores diverging, "rather broad," 

 and terminating "anteriorly [ventrally] in a point" 

 (Foerste), supported by curved plates uniting with the 

 median ridge on the floor of the valve; cardinal 

 process an obscure ridge; median septum extending 

 anteriorly about one-fourth the length of the valve. 

 Adductor scars not impressed on the shell. 



Geologic range. — Early Upper Ordovician of the 

 Ohio Valley. 



Species 



Plectorthis crassiflicata (Foerste) 1910 

 P.sectistriata {\J\nch.) 1879 

 P. sordida multiflicata (Foerste) 1910 

 Rhynchonella sordida Hall 1847 (s)n. Orthis ella) 



^* Schuchert and LeVene in 1929 followed Foerste in 

 reg.irding Cycloccelia as a homonym. However, it appears 

 that, according to the rules, it is not preoccupied by 

 Cydoccela Duf. 1839, as Foerste supposed, and hence the 

 original name given by the latter in 1909 should stand. 



Discussion. — Cyclocaelia forms a compact group 

 of small rhynchonelloid shells having essentially the 

 internal characters of Plectorthis. The author of the 

 genus published only a short analysis of the internal 

 features. The presence of interareas on both valves, 

 the open delthyrium of the ventral valve, and the plect- 

 orthoid interior show that the genus is not a rhyn- 

 chonellid. It differs from Plectorthis mainly in the 

 narrowness of the hinge-line, which has produced the 

 subrostrate condition of the ventral valve. It agrees 

 with that genus in the strength and character of the 

 dental plates and in having the same type of cardinalia, 

 but the cardinal process is much reduced and the whole 

 apparatus more abbreviated. The shell is too thin to 

 have preserved any trace of the muscular impressions. 



The rhynchonelloid form consequent on the nar- 

 rowing of the hinge-line is therefore the fundamental 

 basis of distinction of this group from Plectorthis. The 

 presence of three species and one variety in the genus 

 shows the definiteness of the evolution toward the 

 rhynchonelloid form and is a favorable argument for 

 giving these shells a separate designation. It is obvious 

 that Cyclocaelia is a deviation from the Plectorthis line. 



The development of such a rhynchonelliform shell as 

 Cyclocaelia, and the several independent reappearances 

 of this form in orthid shells, suggest the probability that 

 the Rhynchonellacea may have originated in the Or- 

 thacea, but not in Cyclocaelia, since that genus appeared 

 long after true rhynchonellids had arisen. 



Another rhynchonelliform orthid is the Russian 

 genus Angusticardinia, but this has a different ancestry 

 from Cyclocaelia; moreover, the former has orthoid 

 cardinalia, while those of Cyclocaelia are plectorthoid. 



Subfamily PLATYSTROPHIIN^E Schuchert 1929 



Biconvex, coarsely costate Plectorthidae which either 

 arose in Plectorthis, or came out of the Finkelnhurgia 

 stock. With large subequal interareas, prominent fold 

 and sulcus, and usually a spiriferoid outline. 



Geologic range. — Middle Ordovician to close of 

 Silurian in the northern hemisphere. 



Includes but two genera, Platystrophia King and 

 Mcewanella Foerste. 



Genus PLATYSTROPHIA King 1850 

 PI. 12, figs. 13, 16, 19,23-27 



King, Mon. Perm. Foss., 1850, p. 106. 



Davidson, Brit. Foss. Brach., vol. I, Introd., 1851-185 5, 



p. 103. 

 Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, p. 200, 



pi. 5B, figs. 1-10. 

 Cumings, Amer. Jour. Sci. (4), vol. 15, 1903, pp. 1-48, 



121-136. 

 McEwan, U. S. Nat. Mus., Proc, vol. 56, 1919, pp. 383- 



448. 



