Report of the State Geologist. 



503 



Spirikkk. on the other band, where the mechanical advantage 

 was greater, these impressions on the brachial valve arc often 

 scarcely discernible, and, as L8 often the case among the fossils, the 

 quadruple division of the insertion on this valve is quite obscure. 

 Besides the valvular muscles, there are twopmrs&nd a single 

 unpaired muscle which are attached to the pedicle. One origi- 

 nates on the pedicle-valve at points just outside and behind the 

 diductors; the other pair is attached to the brachial valve behind 

 the posterior adductors, while the unpaired muscle lies at the 

 base of the pedicle, attaching it closely to the pedicle-valve. Of 

 all these muscles for the retraction of the pedicle, it is only the 



Fig. 118.— The muscular system of Crania, a, median unpaired muscle; 6, posterior adductor; 

 c, anterior adductor; d, protractor; e, brachial muscles; /, oral surface; g, brachial fold; 

 /(, arm; i, re. (Joubln.) 



unpaired band that often leaves a discernible scar upon the valve, 

 and in numerous fossil species where the pedicle early became 

 atrophied, even this scar is covered by later depositions of tes- 

 taceous matter. Among some of the inarticulate brachiopods 

 the muscular system is quite as simple as in the articulates. In 

 Crania there are two pairs of strong muscular bands, the one 

 median and close together, the other posterior or cardinal, and 

 further apart. 



The former {anterior adductors) close the valves, the latter 

 {posterior adductors), by their contraction, open the valves to the 

 very limited extent possible among these shells. In addition to 



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