1891.J 
341 
[Hyatt. 
the species describe it as being buried upright in coral masses 
especially Madrepores and as having no byssus . 1 This last of 
course applies only to adults because the young must have had 
such an organ when they first settled on the surface of the 
coral. 
The most remarkable characteristic of Streptopinna is the 
distribution of the nacreous layer. This layer is limited by the 
posterior border of the posterior muscular impression and is 
only slightly broader than the impression and occupies thence 
anteriorly a rapidly and sometimes abruptly decreasing area 
until toward the apices it forms only two narrow, diminishing 
zones on either side of the hinge. The posterior muscular im- 
pressions do not change their positions materially after the adoles- 
cent stage remaining near the apices, an inch to one and a half 
inches distant, even in long shells, and the auxiliary muscular 
impressions could not be detected. This genus is evidently the 
product of the curious tendency formerly acquired by some an- 
cestral species of Atrina, and inherited now by the young of 
living forms, to take up their residence upon the surface of 
growing corals. It can be considered, if one wishes, as an ab- 
errant form of Atrina modified in accord with its peculiar com- 
mensal habitat. 
Sulcatopinna is a new genus proposed for those Carbonifer- 
ous forms having extremely elongated shells, like Aviculopinna, 
with a straight hinge line, umbones approximately terminal, 
valves ridged on the dorsal area. In the typical species the 
valves are bordered dorsally by longitudinal sulcations and raised 
convex zones on either side between the sulcations and the hinge 
line . 2 
The longitudinal ridges appear in the latest nealogic stage 
and early in the ephebolic (adult stage) ; they are apt to be de- 
flected ventrally but the ridges on the convex zone or near the 
hinge continue straight. The hinge line has the peculiar, 
prominent, smooth crests, found also in Aviculopinna. The 
type is S . Jlexicostata sp. Me Coy ; the specimens of this species 
examined were from Kalonga, near Moscow, Russia, and from - 
1 Dufour, Obs. Moll, etc., Ann. Sci. Nat., 2 ser, XIV, 1840, p. 215. 
2 A similar convexity is found in both Palaeopinna and Pteronites at the posterior 
borders of the valves but they have no corresponding convex dorsal areas or sulca- 
tions. 
