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, 1860,f and the Obolus Apollinus, 

 Owen, 1852.T 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 Rupt. N Y. Slatif CiiL. Nat. Hist., p. 133. 18G3. 



t Annual Report Geological Survey Wisconsin, p. 24. 



I Report Geological Survey Wisconsin, Iowa and Minnesota. 



