702 APPENDIX I. 
the muscles evidently separates them from Avicula, to which they are allied in their 
fibrous shells. The posterior muscular impression is very faint, but in some instances 
may be distinguished ; and we therefore remove them from the genus Modiolopsis (Haid), 
which is based on the existence of but a single muscle. 
55. CypricarDIA acuTiFRoNs (Dana).—Thick and much elongate, very oblique, 
(Gervillia-like,) prolonged and broad behind, with the posterior margin orbiculate ; in front 
very short, acuminate. Surface smooth, with a few concentric undulations. Cardinal 
margin straight, about half the length of the shell. Lateral surface convex, and not 
carinate posteriorly, excavate from the beaks downward. Inferior margin concave near 
front, and again more broadly so below the beak. Anterior muscular impression strong, 
situated near the front; posterior indistinct—Length 37 inches ; height of posterior part 
of shell 17 inches; angle between the cardinal margin and the line of elongation of the 
shell, about 82°.—Plate 8, fig. 4 a, 0, different views, natural size. 
District of Illawarra. 
The specimen is partly an exterior cast, with portions of the shell remaining. The 
fibrous texture is distinct. 
Modiolopsis acutifrons, Exped. Foss., Amer. Jour. Sci., 1. Ser., iv. 159. 
56. Cypricarpia impricata (Dana).—Rather thin ; oblong; sub-alate above, sub- 
orbiculate behind; short and obtuse in front and half lower than behind; cardinal 
margin straight and nearly as long as the shell ; surface neatly marked with small con- 
centric ridges; otherwise smooth; sides compressed below the beaks, not carinate pos- 
teriorly. Inner surface of valves towards beaks finely marked with radiate striz.— 
Length 2% inches ; greatest height about half the length.—Plate 8, figure 5, a compressed 
and broken specimen, enlarged one-sixth. 
Harper’s Hill. 
The valve is half a line thick, and throughout is very distinctly fibrous, the fibres 
being easily separable. A line extends from the back of the beak backward, diverging 
a little from the cardinal margin, and thus giving an alate upper border to the shell, upon 
which the faint ridges of the surface are in part nearly vertical. 
Figures 6 and 7, plate 8, represent two specimens, which may belong to the species 
just described. Yet the surface of the cast towards the beak is smooth, and not finely 
radiato-striate ; the alate portion is separated by a more distinct suture, and has its sur- 
face more finely striate. Figure 6. represents a cast, the anterior and posterior parts of 
which are broken; figure 7 is another imperfect specimen, with a portion of the shell 
remaining. The sides are much convex. The beak of the cast is thin, and has almost a 
trenchant summit ; its sides are flattened. The cardinal margin was very long, as in the 
embricata. 
Modiolopsis imbricata, Exped. Foss., Amer. Jour. Sci., ii. Ser., iv. 159. 
57. Cypricarpia arcopEs (Dana).—Thick, subventricose, oblong ; sub-alate above ; 
extremities orbiculate, a little narrower in front; cardinal margin much shorter than the 
shell. Surface nearly as in the ¢mbricata. Sides very convex. Slightly compressed 
anteriorly, Anterior muscular impression vertical, much elongate upward, and forming 
