PLATE XLI. 



(Figures 1 by C. E. Beecher; 2, 8, 29, 30 by G. U. Simpson; 4, 7, s, 11-20, 22-28, 31, 32 by E. EMMONS; 5, (i by R. P. Whit- 

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



Legend : r. Diductoi- scars. p. Cast of the pedicle-cavity. 



a. Adductor scars. 



Genus MERISTINA, Hall. 



Page 65. 



Meristina Maria, Hiill. 



Fig. 1. Tbe youngest shell observed ; showing the foramen which is concealed in the adult condition, and 

 the lenticular valves without fold or sinus. X 5. 



Figs. 2, 3. Two views of an average adnlt 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- val ve ; 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 natui'ally retained by incrustation ; show- 

 ing the form of the cones and the bifurcated loop. 

 Niagara dolomites. BridgejKirt, Illinois. 



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

 primary lamellse. (c.) 



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

 primary lamellse ai-e very long and much stronger than the othei'S ; 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 imperfect I etention. X 5. (c.) 



Fig. 10. A larger but still immature shell which has suffered an injury to the peiipheral 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 ii'regularly contiacte<i cavity, probably without 

 disturbance of function. The spii-al cones are still very depi-esse<l 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.) 



Rg. 11. A restoration of the brachidium, viewed from 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 lamellse, 

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



Pig. 12. A preparation of the interior, in which the brachidium has become detached from the crura and 

 been revolved through neai-ly 180°, almost levei-sing 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 sloj)es. (c.) 

 Niagara group. Waldron, Indiana. 



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

 lamellie and of the deep diductor scare. 



Fig. 15. An internal cast of a lai-ge pe<licle- valve j showing features similar to those in the pi'eceding 

 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. 



t'ig. 17. A cai-dinal 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. 



