30 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



In shells having a spondylium there may be other septa in addition to the median one. This is 

 true notably of Gonambonites, Clarkella, and Y angtzeella, in which the lateral septa probably play 

 a secondary part in the support of the spondylium. In Clarkella the several lateral accessory septa 

 appear to be excessively developed ovarian ridges, and such are common among the syntrophiids and 

 orthids. 



A few genera that have greatly developed diductor scars, especially where these wrap about and 

 enclose the adductor scars, as in Rhipidomella, may develop short, low septa in front of the latter 

 impressions j the septa then separate the antero-median extremities of the diductor scars. These 

 septa are also of no special taxonomic value. 



A few genera are provided with a broad and low median ridge which, besides dividing the 

 diductor impressions, also bears the adductor muscles. This type of septal ridge is seen best in the 

 Schizophoriida;, in which its development can be traced from a low ridge in Pionodema and the early 

 Schizophorias to a high septum in Orthotichia and Enteletes. The same kind of median septum is 

 also seen in certain stocks of the dalmanellids such as Cariniferella, and in these the ridges likewise 

 bear the adductor muscles. 



There is an interesting development of a median ventral septum in Parenteletes. Here the 

 dental plates are similar to those of Enteletes, but are more widely divergent. The median septum, 

 however, has its origin a short distance anterior to the apex, continuing forward and increasing in 

 height in the same direction. But at the anterior end of the septum, just at the point of origin of 

 the shell fold (ventral), there is a small inverted V-shaped chamber, here termed the cella;^^ this 

 chamber evidently acts as an arch to support the dorsal septum above the inner sulcus produced by 

 the external fold. The septum in this genus is further distinguished from that of Enteletes by its 

 expanded, rounded dorsal edge. 



MUSCULATURE 



By means of muscles, brachiopods are enabled to open and close their valves, and to a limited 

 extent to protrude and retract the pedicle and to adjust the position of the shell. In the articulates 

 there are three sets of muscles: the diductor s, which by contraction open the valves; the adductors, 

 which by contraction close the valves; and the fedicle muscles (including the adjustors), which by 

 contraction "serve to alter or adjust the position of the animal as a whole by turning it in various 

 directions.""" All of the muscles traverse the visceral area. The attachment of the muscles to the 

 valves leaves as a rule well marked scars. The diductor muscles which attach to the cardinal process 

 are distinguished as the principal or anterior diductors, while the much smaller pair are the accessory 

 or posterior diductors; the latter are seen in the ventral valve as two small scars posterior to the 

 adductors. In most fossil brachiopods it is only the anterior diductor and the adductor scars that are 

 impressed. 



According to Thomson,^^ 



The pedicle-muscles serve partly to attach the pedicle to the shell, and partly to retract the pedicle or to allow 

 the shell to erect itself. They consist of an unpaired muscle [unknown as such in orthids, where they are always 

 paired] running from the ventral side of the pedicle to the ventral valve, where it determines a transversely elon- 

 gated scar lying posteriorly of the other scars, and, in addition, two pairs known as the dorsal and ventral adjustors. 

 The dorsal adjustors are attached to the hinge-plates or other similar structures in the dorsal valve . . ., while the 

 ventral adjustors form two scars in the ventral valve lying outside the adductor-scars. The pedicle-muscles, not 

 being relatively very strong, do not form strong scars, the median unpaired muscle making the strongest. 



In the Orthacea the ventral muscle field begins small, and progressively expands in later forms. 

 Hall and Clarke saw this long ago, saying:^* 



The entire muscular system on the ventral side of the body, is, in primitive forms, inserted upon the base of 

 the pedicle [delthyrial] -cavity. This is apparent from a study of such a shell as Orthts callactis, where it is per- 

 fectly clear that no muscular bands were attached to the pedicle-valve outside the limits of this strong and con- 



^° From the Latin word for small or extra room. 



^"Parker and Haswell, Text-book of Zoology, vol. 1, 1921, p. 358. 



"Brach. Morph., 1927, p. 30. 



"Pal. N. Y., vol. 8, pt. 2, 1894, p. 338. 



