PLATE XLI. 



(Figures 1 by C. E. Beecher; 2, 3, 29, iO by G. b. bi.MisoN, 4, 7, 6, 11-20, 23-28, 31, 32 by E. Emmons; 5, G by li. P. Wiiir- 



kield; 9, 10, 21 by J. M. Clarke.) 



Legend : r. Diductoi- scars. P- Cast of the iiedicle-cavity. 



a. Adductor scars. 



Genus MERISTINA, Hall. 



rage 05. 



Meristina Maria, Hall. 



Fig. 1. The younf^est shell observed ; showiiifj: the tbi-amen which is concealed in the adult condition, and 

 the lenticular valves without fold or sinus. X ■'J. 



Fiffs. 2, 3. Two views of an avei-ag-e adult individual ; showing' the gibbosity of the valves, the close in- 

 curvature of the beaks and the development of the folil and sinus. 



Fig. 5. The cardinal portion of the bi-achial valve ; showing the median cleft in the hinge-plate, forming 

 an elongate cavity supjiorted by a low median septum. X 2. 



Fi"-. 6. The cardinal portion of a mature pedicle-valve J .showing the open delthyrium from which the 

 deltidial plates have been resorbed, and the thick teeth supported by dental plates. 

 Niagara gi-oup. Waldron, Indiana. 



Fig. 7. The brachidium, viewed from the brachial valve and natuially retained by incrustation j .show- 

 ing the form of the cones and the bifurcated loop. 

 Niagara dolomites. Bridgeport, Illinois. 



Fig. 8. A preparation of the brachidium, showing its relations to the valves, and the great size of the 

 primary lamella;, (c. ) 



Fig. 9. A preparation, showing the conditiciu of the brachidium at a very early stage of growth. The 

 primary lamella: are very long and much stronger than the othei'S ; the spiral cones much 

 depressed and its volutions few and lax. The stem of the loop appears to be simjile at its 

 extremity, but this may be due to imperfect letention. X 5. (o.) 



Fig. 10. A larger but still immature shell which has suffered an injury to the perijAeral 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, S and 11, but the loop seems to have attained its normal condi- 

 thm. 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 x^rimary lamellie, 

 and the mode of attachment of these lamellae to the crura. 



Fig. 12. A preparation of the intei'ior, in which the brachidium has become detached from the crura and 

 been revolved through nearly 180°, almost reversing its relations to the shell but without the 

 disturbance of any of its parts. The median septum 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 directions upon the ventral slopes, (c.) 

 Niagara group. Waldron, Indiana. 



Fig. 14. A partial internal cast of the pedicle-valve; showing the impressions of the conspicuous dental 



lamellfe 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 radi.ate 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. 

 Kig. 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. 



