102 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Genus PORAMBONITES Pander 1830 



PI. 14, figs. 1-15, 20, 21, 24, 27-29 



Pander, Beitr. Geogn. Russ. Reiches, 1830, p. 95, pi. 14, 



fig. 2, pi. 15, fig. 2e {reticulata), pi. 3, fig. 9, pi. 16A, 



fig. 12, pi. 28, figs. 21, 25. 

 Teichert, Neues Jahrb. f. Min., etc., Beil.-Bd. 63, Abt. B, 



1930, pp. 177-246. 

 Subgenus Isorhynchus King, Mon. Perm. Foss., 1850, p. 



112. 



Genolectotype. — Davidson in 1853 chose as the 

 type of the genus Terebratulkes tsquirostris Schlotheim 

 1820, but as this is not one of the species cited by 

 Pander in his work of 1830, it can not be so used. 

 We shall, therefore, follow Teichert, who selects 

 P. reticulata Pander. 



Description. Exterior. — Subcircular to subtri- 

 angular, hinge-line narrow, producing subrostrate 

 shells in some of the species. Cardinal extremities 

 broadly rounded. Lateral profile nearly convexo-plane 

 to strongly biconvex, the dorsal valve always having 

 the greater convexity. Anterior commissure unipli- 

 cate, but the dorsal fold in many species is rather 

 obscure, there being an elongate tongue in the ventral 

 valve. Fold and sulcus developed only in the front 

 half of the valves. Ventral interarea short and nar- 

 row, curved, apsacline; delthyrium open, beak com- 

 monly resorbed or abraded by the pedicle. Dorsal 

 interarea shorter than the ventral, apsacline; notothy- 

 rium open; dorsal beak resorbed in some specimens, 

 so that the pedicle passes through a notch in both 

 valves. Surface ornamented by flat-topped costellae 

 and also by concentric elevated filse or growth-lines 

 which by their union in the interspaces or by alternat- 

 ing with each other produce the effect of pores in the 

 radial depressions. Test fibrous, impunctate. 



Ventral interior. — Delthyrial cavity deep; teeth 

 strong, dental plates well developed, sharply defined 

 in the young and early mature stages, attached directly 

 to the floor of the valve ; in a few species the anterior 

 ends curve slightly and become convergent. In many 

 species, in late maturity and old age the dental plates 

 are enormously thickened, the delthyrial cavity is filled 

 at its back end, and the dental plates are extended for- 

 ward, the adventitious growth coalescing to form a 

 structure simulating a spondylium (^ spondyloid). 

 Ventral muscle impressions lodged in the delthyrial 

 cavity, but migrating forward as the extra testaceous 

 matter is deposited behind. The diductor scars are 

 semielliptical and placed partially on the inner sides of 

 the dental lamellae. The adductor scars occupy a 

 linear track between the diductor impressions. The 

 floor of the umbonal cavities in late mature or old 

 shells is marked by prominent radial ovarian ridges. 



Dorsal interior. — Notothyrial cavity deep, brach- 

 iophore short, sockets wide but shallow; brachiophore 



To these species 

 the name Iso- 

 rhynchus may 

 be applied in 

 a subgeneric 

 sense 



supports prominent strong plates extended directly to 

 the roof of the valve, always separated except when 

 coalesced at the front by a thickening of adventitious 

 shell. Cardinal process absent; diductor scars four in 

 number, attached to the inner walls of the brachiophore 

 supports. Adductor impressions in front of the anterior 

 ends of the brachiophore supports. 



Geologic range. — Throughout the Ordovician of 

 northwestern Europe. Also in the Ordovician of the 

 Himalayas and Australia. 



Species 



Terebratulkes tequirostris Schlotheim 1820 



Pentamerus ventricosus Kutorga 1 846 



Porambonites baueri Noetling 1883 



P.deformatus {'E.ich.viilA) 1829 



P.gigasichimdit 185 8 



P. kuckersensis Bekker 1 92 1 



P.laticaudatus Bekker 1921 



P. schmidti Noetling 1883 



P. /^r^/ior (Eichwald) 1829 



P. wesenbergensisTe'icheit 1930 



Porambonites acutiflicata Reed 1917 



P.altus Pander 1830 



P. broggeri Lamansky 1905 



P.filosa (McCoy) 1846 



P. intercedens Pander 1830 



?. //iwa Sharpe 185 5 



P.farallela Pander 1830 



P. -planus Pander 1830 



P. reticulatus Pander 1830 



P.ribeiro Sharpe 185 5 



P. sinuatus Reed 1915 



P.truncata Pander 1830 



Distinguishing characters. — Porambonites is 

 recognized most easily by its peculiar ornamentation, 

 and on the inside by the strong, divergent or subparallel 

 brachiophore supports and dental plates. 



Discussion. — Porambonites is one of the most 

 unique genera of brachiopods and has many interesting 

 morphologic features. Most important among them 

 are the external shape and ornamentation, the inter- 

 areas, and the internal anatomy of both valves. The 

 shape varies from subtriangular through subpentagonal 

 to subquadrate and subcircular. The species are divis- 

 ible into two groups on the basis of their external form. 

 There is a group of nearly circular forms typified by 

 P. reticulatus {Porambonites s. s.), and the other group 

 of triangular or subpentagonal forms is typified by 

 P. (equirostris or P. deformatus. To this latter group 

 King's term Isorhynchus is applied by us. According 

 to Teichert (chart opposite p. 226) the former charac- 

 terizes Biia-Biii divisions and the other exists from the 

 Echinosphsrites limestone to the Lyckholm of western 

 Russia. In profile these shells are always unequally 

 biconvex, the dorsal valve having the greater con- 



