MYALINA RliDESDALENSIS. 109 



but on anatomical grounds I can find, no reason to separate these groups into two 

 genera. 



There has been some difference of opinion as to whether the shells of Myalina 

 are equivalve or inequivalve. M'Coy and King state that they are inequivalve. 

 De Koninck, who said in his original diagnosis that they were equivalve, came 

 later to the conclusion that some species were equivalve and some not. 



Myalina Redesdalensis, sp. nov., Hind. Plate III, figs. 1—5, 7 — 13; Plate V, 



fig. 1; Plate VII, figs. 1, la. 



Specific Characters. — Shell of moderately large size, triangularly mytiliform, 

 obliquely produced, alate and compressed posteriorly. The anterior portion of 

 the shell is entirely obsolete ; but the shell is expanded from side to side, and 

 acutely bent on itself at a right angle to form an extensive anterior surface, which 

 is slightly convex above, and somewhat concave below from side to side. The 

 anterior surface is oval in outline; the umbones project forwards, forming the 

 extreme upper point, and below the borders approach each other, and unite to 

 form at the lowest point the anterior extremity of the lower border, which is 

 bluntly rounded. The posterior border is obliquely truncate from above down- 

 wards ; in perfect specimens sinuously curved above. The hinge-line is 

 comparatively broad and straight, and meets the posterior border at an angle a 

 little greater than a right angle. 



The umbones are acute,, terminal, twisted inwards, outwards, and forwards, 

 and overlap the hinge-plate. A strong acute ridge, marking off the anterior 

 surface from the lateral, passes from the umbones obliquely backwards and 

 downwards. Posterior to this ridge the shell is rapidly compressed and flattened. 



The shell is very thick and massive in the umbonal region and hinge-plate, 

 gradually becoming very thin posteriorly. 



Interior. — The inner surface of the shell is very smooth. There is a deep, 

 almost conical, smooth cavity in the thick anterior part of the shell, the apex of 

 which is separated from the umbo in an adult specimen by fully half an inch of 

 shell-matter. The hinge-plate is thickened, flat, and longitudinally striated. In 

 front it forms an acute angle with the exterior of the shell, which rapidly becomes 

 more acute as it proceeds backwards. While anteriorly the valve and the hinge-plate 

 are only in contact above, the junction being very thick, posteriorly the valve is 

 twisted on itself, so that the hinge-plate becomes the upper part of its inner 

 surface, the A-shaped cavity which exists anteriorly being gradually obliterated. 

 The hinge-plate is bounded in front by a sharp line directed from beneath the tip 



