LAMELLIBRANCHIATA. 581 
Nuculide.] 
Il. C. gibberula group. 
Shells usually short, thick, with very strongly defined muscular impressions; 
hinge strongly bent, with the series of denticles interrupted beneath the beaks; 
teeth straight or curving outward. 
Species: gibberula Salter, contracta Salter, angela Billings, carinata Ulrich, planodor- 
sata Ulrich, longa Ulrich. 
Ili. C. levata group. 
Shells usually of ovate form, rather thin, with muscular scars moderately dis- 
tinct; denticles converging inward, forming a continuous series in}the Trenton 
species. In the Hudson River forms, however, the series is more or less interrupted 
by the development of a small and undefined pit just beneath the beak. 
Species: Jevata Hall, donaciformis Hall, abrupta Billings, nitida Ulrich, medialis Ulrich, 
scofieldi Ulrich, socialis Ulrich, hartsvillensis Safford, danvillensis, n. sp., retrosa 
Ulrich, filistriata Ulrich, albertina Ulrich, simulatrix Ulrich, tumida, n. sp., 
madisonensis Ulrich, fecunda Hall, calvin Ulrich, mundula, n. sp., perminuta 
Ulrich, nunculiformis Hall, ? hilli Miller. 
IV. C. pectunculoides group. 
Shells subcircular, compressed-convex, beaks very small; hinge plate strong, 
regularly arcuate; teeth in a continuous series. / 
Species: subrotunda Ulrich, circularis, n. sp., pectunculoides Hall, cingulata Ulrich, 
pulchella Hall. 
V. C. recurva group. 
Shells high, the lower half semicircular, the upper subtriangular; hinge plate 
rather strong, bent at nearly a right angle, the (?) anterior part convex, the (?) 
posterior concave; denticles in two distinct series, arranged transversely on the 
plate and therefore at widely different angles on the two. parts of the hinge. 
Species: compressa Ulrich, arstartiformis Salter, intermedia Ulrich, alta Hall, obliqua 
Hall. 
VI. C. logani group. 
Thin gibbous shells, with subcentral large beaks; muscular sears faint: — 
but little arcuate, the denticles in continuous series, bent inward. 
Species: Jlogani Salter, dubia Hall, gibbosa Hall, ? ovata Hall. 
Group I, the typical section of the genus, seems to be strictly confined to Lower 
Silurian deposits and embraces the largest known representatives of the family 
Nuculide. Group II appears to be even more restricted in its range, being unknown 
