MYALINA REDESDALENSIS. Ill 



It is difficult to understand exactly how the hinge-plates were attached to each other. 

 The overlapping umbo would seem to necessitate a fairly thick cartilage. Then 

 was this cartilage attached to the whole of the hinge-plate, or only to the inferior 

 edge ? If we may judge from the conditions which are found in another species 

 of this genus, M. Flemingi, there was little or no separation between the plates ; 

 and this would appear to be the case in the allied genus Naiadites. 



Amongst some specimens, purchased by a grant from the Royal Society, from 

 Palasozoic beds of North America, is a shell, Ptychodesma Knappanum, from the 

 Hamilton group, Indiana, which possesses a striated hinge-plate. This specimen 

 conclusively shows that the hinge-plates were only in contact at the lower edge, 

 and that there existed an elongated V-shaped trench between the plates, probably 

 filled up by cartilage in the recent state. I think that the same condition 

 probably obtained in the larger species of Naiadites and Myalina, because the 

 hinge-plate is placed, in these forms, at such an angle to the rest of the valve 

 that it could only come into contact with its fellow at its lower edge. 



PI. Ill, fig. 7, shows the general contour of the shell, and its rapidly diminishing 

 thickness posteriorly. Fig. 8 represents a fragment which fortunately has pre- 

 served the byssal notch, while Fig. 9 shows the distance of the pallial line from 

 the edge of the shell. Figs. 12 and 13 show the pits for the insertion of 

 the pedal muscles and their relation to the hinge-plate. Fig. 10 shows the 

 change of the striae of growth into marked imbrications as they pass over the oblique 

 ridge. 



This shell is apparently the largest member of the genus, and has a very restricted 

 range. It is known only from one bed in Redesdale, and even here it seems to 

 occur in only one part of the quarry. At this spot, however, it is very plentiful, 

 though, from the fragile nature of the posterior portion of the shell, only frag- 

 ments occur. It was therefore probably gregarious in habit. A few large 

 examples, chiefly in the form of casts, are found in the Limestone of Derbyshire. 

 One of these, in the York Museum, is figured in PL VII, figs. 1 and 1 a. In the 

 list of fossils given in the Geol. Surv. Mem., Geology of the Carboniferous 

 Limestone of Derbyshire, &c, Myalina crassa is quoted as occurring at Park Hill. 

 It is probable that a specimen of M. lledesdalensis was mistaken for that species. 



The only shell that this species is likely to be confounded with is the Naiadites 

 crassa (Mytilus of Fleming). This shell has an anterior lobe, a cardinal tooth in 

 the hinge, and a trifid pit-like anterior adductor scar in the apex of the shell. 

 It differs from the species under description in having no broad anterior surface. 



In its own genus M. Flemingi more closely approaches the species than any 

 other ; but the size, massiveness, and the great expansion of the posterior portion 

 at once serve to distinguish the two forms. 



Professor Lebour mentions the occurrence of M. Flemingi in the Redesdale 



