72 
PALEONTOLOGY OF NEW YORK. 
merge into a pair of elongate submarginal scars, which may be termed the ex¬ 
ternals. They appear to represent the terminal crescent scars in the Trimerel- 
loids,_but cannot, with propriety, be regarded as correlates of the great laterals 
of the opposite valve (see Ford, Walcott, op. cit.). 
The term Dicellomus was proposed in March, 1871, in a paper published in 
advance of the Twenty-third Annual Report on the Condition of the State 
Cabinet of Natural History, for the species which had been previously described 
as Obolella ? polita* the Lingula ? polita, Hall, I860,f and the Obolus Apollinus, 
Owen, 18524 This species was also the third type under Billings’ diagnosis 
of Obolella. Dicellomus was also made to include, as a second type, the Orbi- 
cula ? crassa, Hall, 1847, but this latter species proves to be congeneric with 
Obolella chromatica. The interior characters of the former species are still some¬ 
what imperfectly known, the original illustrations of them which are repro¬ 
duced upon Plate II (figs. 40, 41), being somewhat constructive, but the pedicle- 
valve, while showing an unusually strong cardinal area, has the pedicle-groove, 
the cardinal, lateral and central muscular scars, quite as in typical Obolellas; in 
the brachial valve the apparent correspondence of the scars with those of 
Obolella chromatica and 0. crassa is less marked. Upon this plate are given ad¬ 
ditional figures of the interior of this species, the pedicle-valve (fig. 38) showing 
two very strong centrals, bounded, on the posterior margin, by a thickened, 
triangular area, extensions from which pass between and around the outer sides 
of the scars. The outer posterior margins of this area bear the impressions of 
the cardinal or external muscles, while below the position of the pedicle-groove 
(the cardinal area is not retained on this specimen) lies a pedicle-scar. The 
whole appearance of this interior is strongly suggestive of that in Obolus, but 
may not prove inconsistent with the character of Obolella. In the brachial 
valve, the impressions are more distinctly obolelloid, the long, curved laterals 
taking their origin at the compound central scar. The development of the in¬ 
terior details of the shells of this species is a matter of much difficulty, as the 
* Sixteenth Rept. N. Y. State Cab. Nat. Hist., p. 133. 1863. 
t Annual Report Geological Survey Wisconsin, p. 24. 
| Report Geological Survey Wisconsin, Iowa and Minnesota. 
