NO. 2 BRACHIOPOD SUPERFAMILY STENOSCISMATACEA GRANT IJ 



thest anterior in the last genus. Elevated and sessile spondylia appear 

 throughout the range of the Stenoscismatacea. 



Function. — The interior (concave) side of the spondylium seems to 

 be smoother than the floor of the shell in silicified Permian specimens 

 of Stenoscisma. This smooth area probably was formed by seating of 

 muscles (for evidence, see discussion of musculature) . 



Taxonomic importance. — Spondylia produced by fusion of dental 

 plates are present in many groups (e.g., genera of the Pentameracea, 

 Tetracameridae, Rhynchotetridae, some Orthotetacea) . Presence of a 

 spondylium is not a defining character for the Stenoscismatacea unless 

 accompanied by a camarophorium in the brachial valve ; were a genus 

 to be found containing a camarophorium but not a spondylium, it might 

 be included with the Stenoscismatacea. 



Insertion. — The median septum of the spondylium is inserted into 

 the valve floor like a wedge ; its fibers are not continuous with those of 

 the floor, and it is not derived from the floor. This peculiar insertion 

 was described by Licharev (1936, p. 57, fig. 1) and is shown here in 

 plate 6, figs. 4, 4a; plate 17, fig. 5a; and figures 11-13 and 31. It is in 

 direct contrast to the normal form of median septum where the fibers 

 of the shell wall can be traced continuously from the valve floor to the 

 side of the septum. The normal form of growth of septa was described 

 for billingsellids and clitambonitids by Kozlowski (1929, p. 121-132, 

 figs. 37-38) and illustrated by photographs of transverse sections of 

 orthoids by Schuchert and Cooper (1932). Both of these works also 

 illustrate the manner of growth of the septum of the spondylium in 

 pentameroids, where it wedges into the shell material of the valve 

 floor as in the Stenoscismatacea. 



The similar manner of insertion of the septum of the spondylium of 

 the pentameroids and the stenoscismataceans might argue for close 

 phylogenetic relationships between the two groups. Earlier workers 

 (e.g., Girty, 1909) classified the stenoscismatids with the pentameroids, 

 largely on the basis of similarity of the camarophorium and the spon- 

 dylium with the elevated spondylia of the pentameroids. Later authors 

 (e.g., Kozlowski, 1929; Muir-Wood, 1955) include the Stenoscis- 

 matacea with the Rhynchonelloidea because of their crura and char- 

 acteristic rhynchonelliform pedicle beak (see discussion of classifica- 

 tion). 



The spondylium of the Pentameroidea is different in shape from the 

 spondylium and camarophorium of the Stenoscismatacea and, accord- 

 ing to Schuchert and Cooper (1932), pentameroid cardinalia are not 

 homologous to those of the Rhynchonelloidea. Similarity in the man- 

 ner of insertion of septa in the two groups may reflect mechanical rather 

 than phylogenetic causes. Cross-sections of a Meekella in which the 



