588 Systkmatic Paleoxtolooy — Upper Devonian' 



Description. — •"'Shell robiif;!, i^uljorbieular or cnoid : width greater or 

 less than the length; radiatingly costate and concentrically lamellose or 

 spinose; hinge-line often nearly straight, a little less than the width of the 

 shell. 



" Ventral valve depressed-convex, becoming more convex in the npper 

 part; nearly flat and often a little concave towards the lateral margins, 

 and cardinal extremities depressed or hi'oadly sinuate in front; beak 

 abruptly rounded ; apex truncate and perforate, closely appressed and 

 overlapping the umbo of the opposite valve. 



" Dorsal valve convex, becoming gibbous in old shells, flattened or 

 slightly concave towards the cardinal angles, regularly curving to the 

 sides and basolateral margins, and a little elevated in front, but without 

 any distinct mesial fold. 



" Surface marked . by strong rounded radiating costge bifuracting at 

 unequal intervals, which are much stronger in the middle of the valve, 

 and become obsolete or appear as gentle undulations toward the cardinal 

 angles. In the middle of the valves there are about seven or eight of these 

 costfe in the space of half an inch. The shell is also marked by strong- 

 concentric lamellse, which are often about a line apart. In perfect shells 

 these lamellae, at the crossings of the costfe, are often produced into 

 tubular spines, which, when worn off, leave the ordinarily lamellose 

 surface. The spaces between these projecting lamcllse are marked l)y 

 fine thread-like striae. 



" In the separated valves, the hinge-line is often nearly straight, the 

 muscular area of the ventral valve is short and broad, the length from the 

 a2)ex being about equal to the width. There is a slight thickening of the 

 shell at the base of the rostral cavity. The surface around the muscular 

 area is papillose, and limited by a thickened border, except in front, 

 where it is discontinued. Fine vascular markings are sometimes visible 

 near the margin. In the dorsal valve there is a thickened septum in the 

 upper part of the muscular area,. 



" The spires of full-grown individuals show about fifteen turns in 

 each." Hall, 18G7. 



Length 22 mm.; width 23 mm. 



