158 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



duplex septum of Pentamerus, but is single as in Ck- 

 tambonites or Vellamo, which occasionally also have 

 the septum fractured medially. The spondylium is 

 supported for practically its whole length and there are 

 generally two lateral septa as in some species of Clark- 

 ella. At its front end the spondylium hangs free of the 

 septum, which is prolonged forward as a ridge on the 

 floor of the valve. Between the main septa on the 

 floor of the valve are numerous accessory radiating 

 ridges, such as are common in Syntrofhina and Clark- 

 ella. These radiate from each of the umbonal cavi- 

 ties, the larger ones forming the septa. They are 

 interpreted by Kolarova as ridges or septa between the 

 diductor muscles. According to our view, they are 



Fig. 23. — Yangtzeella foloi (Martelli). Section cut 

 through an adult specimen, showing septa of both valves 

 and notothyrial chamber of the dorsal valve, x about 4. 



for attachment of the ovarian bodies, and so far as our 

 knowledge goes, the ventral muscle marks in spon- 

 dylium-bearing shells are always located on the upper 

 or dorsal surface of the spondylium, and such shells 

 never have additional muscle-scars on the valve floor. 

 Low ridges upon the spondylium of Yangtxeella sug- 

 gest that the muscles here were also attached to the 

 upper surface of the spondylium. 



The dorsal valve is especially interesting and quite 

 unlike any other brachiopod except Clarkella. There 

 is a narrow, spoonlike platform supported by two 

 lateral septa, and two or more accessory ones. After 

 about 3.5 mm. in a specimen 20 mm. long, the spoon 

 terminates, but the lateral septa persist and are overlain 

 by horizontal plates or shelves that may or may not be 

 supported by septa. The brachiophores consist of 

 rather long processes extending into the valve from 

 the extremity of the brachiophore plates, as in 

 Syntrofhina. 



This shell was originally referred to Triflesia and in 

 the 1929 classification of Schuchert was placed in asso- 

 ciation with this and allied genera. Yangtxeella, how- 

 ever, has no forked cardinal process, and besides, the 

 presence of a spondylium and cardinalia like those of 

 Clarkella shows that it belongs with the Syntrophiidae, 



where Miss Muir-Wood placed it.^ It is closest to 

 Clarkella in a broad sense, having many septa in each 

 valve. However, Clarkella does not have a spoon- 

 shaped dorsal plate or lateral processes such as occur in 

 Yangtzeella. 



Family SYNTROPHIID^ Schuchert 1896 



Smooth biconvex Syntrophiacea having well devel- 

 oped interareas, a spondylium simplex, and a cruralium 

 simplex. 



Geologic range. — Lower Ordovician. 



Embraces but one genus, Syntrofhia Hall and 

 Clarke. Walcott has referred to this family his 

 Swantonia, a genus whose taxonomic position is wholly 

 unknown, but we leave it here as we are not able to 

 place it. 



Genus SYNTROPHIA Hall and Clarke 1891 

 PI. 15, figs. 3, 27-29, 32 



Hall and Clarke, Pal. N. Y., vol. 8, pt. I, 1891, p. 270; 

 pt. 2, 1893, p. 216, pi. 62, figs. 1-10. 



Genoholotype. — Triflesia lateralis Whitfield 

 1886, Bull. Amer. Mus. Nat. Hist., vol. 1, no. 8, p. 

 303, pi. 24, figs. 9-11. 



Description. Exterior. — Transversely oval, hinge- 

 line narrower than the greatest width, cardinal ex- 

 tremities obtuse; lateral profile biconvex; anterior 

 commissure unipHcate; fold and sulcus usually shal- 

 low; areas on both valves; ventral interarea apsa- 

 cline, delthyrium open; dorsal interarea anacline, 

 notothyrium open. Surface marked only by fine con- 

 centric lines of growth. Test fibrous, impunctate. 



Ventral interior. — "Teeth small" ; dental plates 

 uniting with a median septum to form a spondylium 

 simplex that is free at its anterior end. Muscle con- 

 fined to the spondylium. 



Dorsal interior. — Much like the ventral, the brach- 

 iophore supports converging and uniting with a 

 median septum to form a cruralium. Pallial trunks 

 radiating from the muscle area as in Syntrofhina. 



Geologic range. — The only known species is the 

 genotype, which comes from the Lower Ordovician 

 (upper Beekmantown) of Vermont. 



Distinguishing characters. — The presence of a 

 spondylium and a cruralium is the chief distinguishing 

 feature. 



Discussion. — When Hall and Clarke based Syn- 

 trofhia on Triflesia lateralis, they were struck most by 

 the external form and the presence of a spondylium. 



>Zool. Record, 1926. 



