118 PALAEONTOLOGY OF NEW YORK. 



axial line. The first one-third of this tnbe is swollen and thickened, but 

 thenceforward it passes, with parallel edges, to the anterior margins, where it 

 is probably closed, though usually broken. Dorsal valve more depressed, its 

 greatest convexity being in the center. Cardinal area as in the opposite valve, 

 but narrower. A closed tube, beginning in a swelling just in front of the car- 

 dinal line, is continued [from the center of the shell] to the anterior margin 

 as an open channel. Muscular scars as in Obolus and Helmersenia, but more 

 sharply defined." (Pander, he. cit.) 



Type, Orbicula Buchi, Verneuil. 1845. Grol. de la Russ-. de I'Europe et des 

 mont. d'Oural, p. 288, pi. xix, figs. 1 a, b, c. 



This most peculiar genus presents a close alliance in its muscular impressions 

 to Obolus, perhaps more nearly to the Neobolus of Waagen, but in its interior 

 closed tubes, that of the pedicle-valve communicating with the external fissure, 

 its relationship to the Siphonotretids is demonstrated. Too little is known of 

 the permanent characters of the fossil to form any reliable conclusions in regard 

 to its proper association, but from the foregoing description it would appear to 

 present the anomalous character of a Siphonotretid in which the pedicle-tube 

 is in a condition of atrophy, compelling the pedicle to pass between the 

 valves, over the cardinal area, as in Obolus. The presence of a caecal tube in 

 each valve, if ever devoted to the passage of the pedicle, is altogether un- 

 precedented. 



In regard to the propriety of the generic term Keyserlingia, it may be 

 observed that although Pander's description and illustration are given without 

 specification of the typical species, the genus was founded on the " Orbicula, 

 Murch., Vern,, Keys. Geol. de la Rus.sie, 1845, vol. ii, pag. 288," i. e., Orbicula 

 Buchi, not 0. reversa, Vern., which has been quoted by Dall* as the type-spec- 

 ies, but which is described on page 289 of that work. The genus Orbicella, 

 D'Orbigny, 1847, was evidently founded to include the class of shells for which 

 Sharpe, in the same year, proposed the name Trematis. It does not appear, 

 however, that any example was cited with the first use of this term in the 



'■ Biillt-tiii No. S, U. S. National Museum, p. 39. 1877. 



