DEVONIAN FOSSILS 105 
Interarea 4-5 mm. high in the middle; forms a somewhat acute 
angle with the plane of the exterior shell surface. Teeth supported 
by strong dental plates which continue anteriorly into thick ridges 
which completely enclose the muscle field. The dental lamellae and 
ridges are not vertical, but rise from the floor of the shell inwards 
at an angle. Muscle field inverted heart-shape, 2-5 em. long down 
the midline and with greatest width of 2-8 em. Adductor impres- 
sions of oval outline, about 1 em. long and 0-5 em. wide, divided 
by a broad low median septum which becomes higher and sharper 
on the anterior side of the impressions. Adductor scars nearly 
smooth, and surrounded by diductor scars radially strongly 
furrowed, and with concentric fine lines and rugae. Anterior 
margin somewhat crenulate. The remainder of the interior of 
the ventral valve is marked with costellae (especially near the 
margin), with concentric ridges, and with very numerous fine 
papillae irregularly disposed. The papillae tend to be bluntly 
conical in shape, but a great number are irregular. 
Interior of dorsal valve shows massive cardinalia. Strong blade- 
like septum 1-5 mm. high at the posterior end, but decreasing in 
height and width anteriorly. Septum reaches about 2-8 em. from 
the hingeline. Large crural bases about 1-5 mm. wide, disposed 
more or less parallel with the hingeline. 
Description of Paratype. The dorsal valve of the holotype, 
being in situ, is naturally partly hidden at the posterior end by 
the big umbo of the ventral valve. A paratype consisting of a 
steinkern of a dorsal valve is therefore presented in order to 
elucidate the umbonal structures. This steinkern shows a very 
large cardinal process with two large discrete prongs, each divided 
posteriorly into two small knobs or processes. The prongs are 
splayed apart (Pl. IL, Fig. 2), are 3-5 mm. wide, and merged 
anteriorly into the median septum, which in this specimen is 
prominent for 2 сіп. but continues less conspicuously to a point 
3 em. from the umbo. At the umbonal end, the septum is 1 mm. 
wide and 1-5 mm. high. The size of the cardinal process means it 
would project some distanee into the umbonal cavity of the ventral 
valve, but the holotype steinkern shows that this was capacious. 
'l'he erural bases are strong, 2 mm. wide, and stand parallel to the 
hingeline. The cardinal and brachial processes are fused into one 
massive plate. Palintrope obsolete. 
Comment. Points of ecological interest are: 
(1) The presence of the two valves together in the holotype is 
evidence of comparatively quiet waters, an inference already 
made from the composition of the fauna as a whole. 
