502 



Forty-fifth Report on the State Museum. 



margin. 



In species where the cardinal process is feebly developed, 

 as in the plicated Spirifers, these 

 muscles were very powerful, as is 

 evinced by the deeply excavated scars 

 they have left ; so, also, in Orthis 

 (Rhipdomella) and Hippaeionyx 

 (Plates 12, 16) the scars may cover a 

 great part of the inner surface of the 

 valve, extending almost to the anterior 

 y ^^Miiffiii#^ v ' margin : while in other species where 



Fig 116.— Orthis musculosa. Inte- . 



nor of a pedicie-vaive; a, adductors; the cardinal process was larger and the 

 r, diductors;p, pedicie-muscie. leverage more equab i e? these scars are 



much feebler and of less size (Triplegia, Streptorhynchus, 

 Stringocephalus ; Plates 17, 18). In the posterior part of the 

 muscular region of the pedicle-valve are two much smaller mus- 

 cular bands (accessory diductors, see figure 1 14) which are inserted 

 on the cardinal process behind the principal diductors. The scars 

 of this pair are very rarely seen among the fossils, and it is 

 doubtful if they existed among the early genera, such as Orthis, 

 Strophomena, Siropheodonta, etc., where the principal diductors 

 attained such great size. 



The adductor or closing muscles have a double origin on the 

 pedicle-valve, leaving two elongate scars on either side of the 

 median line, and lying between the scars of the diductors. They 

 traverse the interior cavity almost perpendicularly to the valves, 

 each muscular band dividing, and their insertion on the brachial 

 valve being therefore quadruple. Their divisions on that side of the 

 animal are known as anterior and posterior adductors. The cen- 

 tral position of these muscles gives them 

 a great mechanical advantage in closing the 

 valves and they are therefore less powerful 

 than the diductors; among the fossils they 

 are often obscurely defined. The size and 

 definition of these scars will always be found 

 in direct relation to the amount of work 

 required of the muscles. Thus in Stropheo- 

 donta and Productus their insertion upon 

 the brachial valve is so far back that they 

 arc at a disadvantage in moving (his valve; 

 and in both of these genera, the scars are usually strong. In 



5-1 



Fig. 117.— I n te r ior of 

 brachial value of Or this 

 Vanuxeuti. j, cardinal 

 process; a, d, anterior 

 and posterior adduc 

 tors. 



