DE. T. DAVIDSON ON EECENT BEACHIOPODA. 



211 



Fig. 22. 



A. 



B. 



member passing across the middle to reverse sides of the shell. One pair {h) has its members 



confined to nearly the central region. The odd muscle {(/) occupies the umboaal cavity. 

 Lateral muscles.— In. the dorsal valve one pair [j), which may be termed the anterior is 



attached to the median plate, a member 



passing from each of its sides* to the corre- 

 sponding one in the opposite valve." [This 



median plate or rounded ridge is very pro- 

 minent in some specimens and species, 



especially so in Lingiila Mtirphiana.'\ " The 



second or outside pair [l] in the ventral valve 



has each member passing from the outer 



side of the central muscles to the same side 



in the dorsal valve, close to the posterior 



half of the ante-latero-parietal, and in the 



widest part of the splanchnoccele. The third 



or middle pair {k) springs from the ventral 



valve between the central muscles, each 



member passing to its corresponding side in 



the oj^posite valve, increasing much in size 



in the passage, and l^ecoming inserted in 



front of, and inwardly to, the attachments of 



the muscles belonging to the last pair." 



These muscles leave deepish oval-shaped 



scars in the interior surface of the valve, and 

 ., are generally easily recognizable. 



" Transmediaii muscles {i). — In the dorsal 



valve both members of this pair are im- 

 planted in the widest part of the splanchno- 

 ccele, one on the inner side of, and immediately adjacent to, the termination of the 

 laterals k and I. Passing backward, each member crosses diagonally to the reverse side 

 of the ventral valve ; but while one preserves its unity, and terminates by inserting 

 itself near the middle of the left post-latero-parietal, the other is divided in its passage, 

 a division becoming inserted near the posterior, and another near the anterior, end of the 

 opposite and corresponding parietal. The divided member embraces the undivided one." 

 The lateral muscles do not always leave well-marked impressions in the interior of the 

 valves ; they are only seen occasionally on very well-preserved specimens. " It may be 

 assumed," continues Prof. King, " that the central and umbonal mviscles ((/, li) elFect the 

 direct closing and opening of the shell, and that the laterals {j, k, I) enable the valves to 

 move forward or backward on each other : but with respect to the transmedians (/), it is 

 difficult to conceive otherwise than that they allow the similar extremities (the rostral) of 

 the valves to turn from each other to the right, or the left, on an axis subccutrally 

 sitnated — that is, between the anterior attachments of these muscles a little behind the 

 medio-transverse line of the dorsal valve." 



L!)ii/>da anatina. 



A. Interior of tho ventral or pedicle valve. 



B. Interior of the dorsal valve (after King) : fj, umbo- 



nal muscular impressions (open valves) ; h, central 

 muscles (close valves) ; i, transmedian or sliding 

 muscles : 6, parietal band : ./, Ic, 1, lateral muscles 

 (./, anteriors ; I; middles ; 1. outsiders), enabling 

 the valves to move forward and backward on each 

 other. 



