MYALINA COMPRESSA. 123 



Myalina compkessa, sp. nov., Bind. Plate V, figs. 2, 3. 



Specific Characters. — Shell much compressed, flattened, and expanded, sub- 

 quadrate ; anterior end quite obsolete. The anterior border of the shell is rounded 

 and not well defined, arcuated and sinuously curved, concave above, convex below, 

 passing insensibly into the broadly curved inferior border. The posterior border 

 is convex in outline, joining the hinge-line at an obtuse angle. The hinge-line is 

 straight, shorter than the transverse diameter of the shell. The umbones are 

 pointed, anterior, and project forwards. The anterior part of shell is somewhat 

 swollen, and forms an anterior surface, which is convex from side to side, and 

 concave from above downwards. The gibbose portion of the shell is very limited ; 

 the remainder is flattened and expanded posteriorly. The right valve is much 

 flatter than the left. 



Interior. — The umbones of casts are pointed and separated by a wedge-shaped 

 trench. The posterior adductor scar is large, situated close to and just below the 

 posterior end of the hinge-line. Pallial line entire. 



Exterior. — Not known. The greatest thickness of the shell is anterior and just 

 below the umbones. 



Dimensions. — PI. V, figs. 2, 3, measures — 



Vertical diameter . . . .18 mm. 



Horizontal diameter . . . .18 mm. 



Thickness of cast -. . . .6 mm. 



Locality. — A marine band in the Coal-measures of South Staffordshire. 



Observations. — This species is founded on certain casts in the possession of 

 Mr. Johnson, of Dudley. They are typical of the genus, showing the space in 

 front between the umbones (for the thickened hinge-plate), which gradually 

 narrows and becomes linear. The posterior adductor muscle-scar is well marked, 

 very high up, and close to the hinge-line — a condition very different from that which 

 obtains in all the other species of which the interiors are known. This character 

 has caused me to consider the species as separate from that described by Messrs. 

 Meek and Worthen as Myalina meliniformis in the ' Proceedings of the Chicago 

 Academy of Science,' vol. i, p. 19, 1866, and also in ' Geological Survey of Illinois,' 

 vol. ii, ' Palaeontology,' p. 343, pi. xxvii, fig. 3, where the posterior adductor 

 scar is shown to be in the normal situation. One of the specimens which I figure 

 (PL V, fig. 3) has very faint indications towards the lower part of the left 

 valve of surface-markings. These are concentric, with occasionally stronger lines 

 standing out in relief. 



It is unfortunate that the exact horizon where these shells occur is not known, 

 and also that testiferous examples are not to hand. 



