344 CARBONIFEROUS LAMELLIBRANCHIATA. 



this or any other species of the genus possessed a byssus, but the marked 

 sulcation anterior to the ridge very closely resembles the byssal sulcus in shells 

 possessing that organ, but it is probably an ancestral relic pointing to descent 

 from some byssiferous ancestor. 



Mytilomorpha angulata, sp. nov. PI. XXXVIII, figs. 12 — 18. 



Specific Characters. — Shell large, triangularly cuneate, much swollen, markedly 

 angulate and carinate, very inequilateral and oblique. The anterior end is very 

 short and narrowed from above downwards, much compressed, its border rounded 

 and small in extent. The inferior border is produced, sinuous, directed down- 

 wards and backwards, and becomes convex posteriorly. The posterior border is 

 truncated obliquely from above downwards and backwards, nearly straight for 

 the upper two-thirds, but below it is gradually rounded forwards to meet the 

 inferior border at a well-marked angle, slightly larger than a right angle. The 

 hinge-line is arched and depressed in front, but posterior to the umbones is 

 straight and produced. The umbones are small, pointed, contiguous, twisted 

 forwards and downwards, and reach almost as far as the anterior edge of the shell, 

 above which they are raised. The lunule is deep and excavated, and the 

 escutcheon is long and wide, bounded externally by a marked elongate angular 

 fold. Passing backwards and downwards from the umbo to the posterior inferior 

 angle is a very high acute riclge, which is gently sinuous and often everted in a 

 downward direction, and divides the valves into two unequal parts — an anterior 

 and lower, which is elongate and narrowed ; an upper and posterior, which is 

 shorter and broader. These portions are placed at an angle of about 45° to each 

 other. To attain this position the lower portion becomes twisted on itself, the 

 extreme anterior end being flat and vertical. The lower portion of the valve is 

 compressed and hollowed so as to be slightly concave on section, the lower part 

 of the valve being bent rapidly inwards to meet its fellow; the upper portion, 

 forming the dorsal slope, is regularly but slightly convex. 



Interior. — The anterior adductor muscle-scar is large, deep, and triangular, 

 •placed within the small anterior end, the greater part of which it occupies. The 

 scar is bounded behind and below by a ridge of shelly material. The posterior 

 adductor scar is large, rounded, rough, placed well within the margin of the shell, 

 and some distance below the hinge-line in the hollow of the posterior slope. The 

 pallial line is entire and remote from the margin. 



The hinge has not been clearly seen. 



The interior of the shell is smooth, the anterior portion showing behind the 



