PLATE XLl 
(ITigures 1 by C. E. BEECHEk; 2, 3, 29, 30 by G. ii. Semi-sojs; 4, 7, 8, ii-20, 22-28, 31, 32 by E. Emmons; 5, 0 by R. P. M’lilT- 
field; 9,10, 21 by J. M. Clarke.) 
Legend: r. Diductor scars. p. Cast of the pedicle-cavity, 
a. Adductor scars. 
Genus MERISTINA, Hall. 
Page 65. 
Meristina Maria, Hall. 
Fig. 1. The youngest shell observed ; showing the foramen which is concealed in the adult condition, and 
the lenticular valves without fold or sinus. X S. 
Figs. 2, 3. Two views of an average adult individual; showing the gibbosity of the valves, the close in¬ 
curvature of the beaks and the development of the fold and sinus. 
Fig. 5. The cardinal portion of the brachial valve; showing the median cleft in the hinge-plate, forming 
an elongate cavity supported by a low median septum. X 2. 
Fig. 6. The cardinal portion of a mature pedicle-valve ; showing the open delthyrium from which the 
deltidial plates have been resorbed, and the thick teeth supported by dental plates. 
Niagara group. Waldron, Indiana. 
Fig. 7. The brachidium, viewed from the brachial valve and naturally retained by incrustation ; show¬ 
ing the form of the cones and the bifurcated loop. 
Niagara dolomites. Bridgeport, lUinoi.<i. 
Fig. 8. A preparation of the brachidium, showing its relations to the valves, and the great size of the 
primary lamellEe. (c.) 
Fig. 9. A preparation, showing the condition of the brachidium at a very early stage of growth. The 
primary lamellae are very long and much stronger than the others ; the spii-al cones much 
depressed and its volutions few and lax. The stem of the loop appears to be simple at its 
extremity, but this may be due to im[)erfect letention. X 5. (c.) 
Fig. 10. A larger but still immature shell which has suffered an injui-y to the periphei-al growth of 
the valves on one side. This obstruction has produced a deformation of the spiral cone on 
that side, which has conformed itself to the irregularly contracted cavity, probably without 
disturbance of function. The spiral cones are still very depressed in comparison with the 
adult condition seen in figs. 7, 8 and 11, but the loop seems to have attained its normal condi¬ 
tion. X 2. (c.) 
Fig. 11. A restoration of the brachidium, viewed fi-om the pedicle-valve, a portion of the cones being re¬ 
moved to show the loop, the great divergence of the umbonal curves of the primary lamellie, 
and the mode of attachment of these lamellae to the crura. 
Fig. 12. A preparation of the interior, in which the bi-achidium has become detached from the crura and 
been revolved through neai'ly 180°, almost reversing its relations to the shell but without the 
disturbance of any of its parts. The median seiitum of the brachial valve remains in its nor¬ 
mal position, (c.) 
Fig. 13. Lateral view of one of the spiral cones in its normal relation to the valves; showing the slight 
compression in two dii ections upon the ventral slojies. (c.) 
Niagara group. Waldron, Indiana. 
Fig. 14. A partial internal cast of the pedicle-valve; showing the impressions of the conspicuous dental 
lamellae and of the deep diductor scars. 
Fig. 15. An internal cast of a large pedicle-valve ; showing features similar to those in the preceding 
figure, and radiate vascular markings over the pallial region. 
Fig. 16. The internal cast of a smaller shell which had been greatly thickened in the umbonal region of the 
pedicle-valve; showing the impression of the strong muscular scar and the vascular sinuses 
diverging from about its margins. 
Fig. 17. A cardinal view of the same specimen ; showing the filling of the pedicle-cavity, and the position 
of the median septum of the brachial valve. 
Niagara dolomites. Bridgeport, Illinois. 
