8. W. Ford — Lower Silurian Brachiopoda. 467 



Generic characters. — Shell thin, calcareous, inarticulate, lon- 

 gitudinally ovate or sub-circular, convex. Ventral valve with 

 a solid beak and a minute area, which, in the typical species, is 

 grooved for the passage of the pedicle as in Obolelia. Muscular 

 impressions in the ventral valve six, one pair situated close to 

 the cardinal edge, one on either side of the median line; a 

 second, smaller pair, placed directly below the former; and 

 outside of the latter a third pair of large elongate orsubreniform 

 impressions, converging forward. Beneath the rostrum there 

 is a prominent spoon-shaped pit or chamber separating the 

 above mentioned impressions, with which the groove of the 

 area is confluent. In the dorsal valve there are also three pairs 

 of impressions disposed in nearly the same manner with those of 

 the ventral valve. The dorsal valve is not known to possess 

 an area. The surface is concentrically striated. 



If we compare the interior of the ventral valve of Billingsia 

 desiderata with that of Obolelia crassa (see figs. 1 and 2) we shall 

 find that they present several important differences. They 

 coincide in this, that they each possess three pairs of muscular 

 impressions; but the central pair in B. desiderata sustain a dif- 

 ferent relation to the cardinals than those of 0. crassa, while 

 the size, form and disposition of the laterals are plainly distinc- 

 tive. Moreover, the large spoon-shaped cavity beneath the 

 rostrum has not, so far as I am aware, any exact homologue in 

 any known species of Obolelia. Its large size and forward ex- 

 tension forbid our interpreting it as the homologue of the pedicle 

 pit of Obolelia, although the pedicle very probably terminated 

 in it. 



Obolelia? ambigua Walcott (Paleontology of the Eureka Dis- 

 trict, p. 67, pi. 1, figs. 2 a, b, c, 1884) has also a thin shell, with 

 muscular impressions similar to those of B. desiderata, and will 

 probably fall into the same genus. Obolelia pretiosa Billings and 

 O. Ida Billings (Palaeozoic Foss., vol. i, pp. 68 and 71) are also 

 thin-shelled species; and, when their internal structure shall 

 have been determined, will very possibly prove to be generic- 

 ally identical with B. desiderata. All of these species occur 

 low down in the Silurian rocks, and apparently at about the 

 same geological horizon ; B. desiderata and 0. Ida having been 

 described from the Levis formation of the Canadian geologists, 

 0. pretiosa from the Sillery, and O.f ambigua from the lower 

 portion of the "Pogonip group" of Central Nevada. 



It is possible that Billingsia should be regarded as only a 

 sub-genus of Obolelia. My present impression however, is, that 

 it probably sustains about the same relation to that genus, that 

 the genus Olenellus does to Paradoxides. 



Schodack Landing, N. T., March 2d, 1886. 



