6 * Prof. W. King on some 



inserted near the posterior, and another near the anterior, end 

 of the opposite and corresponding parietal. The divided 

 member embraces the undivided one. 



The umbonal (figs. 1, 2, 3, 4, & 5, g) and central (h) muscles 

 do not require any further description. 



Lingula differs widely in its muscular system from most 

 other Palliobranchs. The umbonal, if, as suggested by some, 

 it really be two muscles that have become united, may be 

 homologous with the posterior adductors in Discina. There is 

 some probability that the centrals correspond to the only pair 

 of adductors or valvulars occurring in the Terebratulids and 

 Rhynchonellids. The laterals appear to have little in common, 

 except their distinctive position, with either the pedicle or the 

 cardinal muscles in the last-named families. As to the 

 transmedians, they are essentially distinct from every muscle 

 of other genera, even the allied Discina * — none in the latter 

 ever connecting the two valves by their reverse or opposite sides. 



It may be assumed that the central and umbonal muscles 

 (g, h) effect 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 subcentrally situated — that is, between the anterior at- 

 tachments of these muscles a little behind the medio-transverse 

 line of the dorsal valve "f- Still there are two important 

 points, seemingly opposed to this view, that require to be 

 considered. 



Thus the position of the umbonal muscle appears to be 

 capable of preventing any lateral displacement of the valves at 



* I may on a future occasion describe the myology, and some other 

 characters, of this interesting shell, as there is much to be added to, or 

 amended in, the description given of it in papers already published. At 

 present I may merely mention that the two small muscles inserted in the 

 convex valve of Discina, between the adductors, appear to be the homo- 

 logues of the cardinals, discovered by Quenstedt in Rhynchonella (pub. 

 1835), and by myself, without knowing the latter fact, in Waldheimia 

 (pub. 1848). It would thus appear that Discina is closer in this respect 

 to the Terebratulids and Rhynchonellids than to Lingula, in which 

 these muscles are absent. 



t Cuvier, in the ' Memoires du Museum,' vol. i. p. 69, 1802, was the 

 first to notice the peculiar muscular character of Lingula. The muscles 

 acting separately, he states, would be able to slide the valves in all 

 directions ; but he does not mention the precise mode of action of the 

 transmedian muscles. The " sliding " motion of the valves has been 

 strongly contested of late years ; nevertheless, as will shortly be seen, the 

 Cuvienan view is undoubtedly the true one. 



