PLATE LV. 



(Figures 1 by J. M. Clarke; 



, 11, 1-2, 15-17 by G. B. Simpson; 3-9, 18-21, 23, 26, 27 by B. 1'. \Viirn-n;i.D; 10, 13, H, 

 22, 24, 25 by E. Emmons.) 



Legend : P. Pedicle passage. 



p. Pedicle cavity, 

 dp. Deltidial plates, 

 hp. Hinge-plate. 



c. Crural lobes. 



h. Dental sockets. 



si\ Ridge in dental sockets. 



a. Adductor scars. 



n. Diductor scars. 



o. Ovarian markings. 



V. Vascular trunks. 

 v'. Secondary vascular sinuses. 



Genus ATRYPA, Dalman. 



Page 163. 



Atrypa ueticularis, Linii6. 



Fig. 1. Dorsal view of the youngest individual observed; showing the slight convexity of the brachial 

 valve in tlie umbonal region, its general depression anteriorly, low median sinus, few plications, 

 erect beak of pedicle- valve, triangular delthyiium and incipient deltidial plates. X 10. 



Fig. 2. Exterior of the pedicle-valve of a mature imlividual from the same locality ; showing the exten- 

 sions of the concentric lamella?. 



Niagara group. Waldron, Indiana. 



Figs. 3, 4. Dor.sal and profile views of an elongate, finely plicate and gibbous shell, without strong con- 

 centric growth lines. 



Lower Helderberg group (Shaly limestone). Near Clarksville, N. Y. 



Figs. 5, 6. Dorsal and jirofile views of an orbicular, subequally biconvex, finely plicate shell. 



Fig. 7. The interior of a pedicle-valve ; showing the broad pedicle cavity, widely separated teeth, pedicle, 

 a<iductor and diductoi' mufcular scars, ovarian markings and crenulated ante-lateral margins. 



Fig. 8. The interior of a brachial valve ; showing the structure of the hinge-plate, dental sockets and the 

 muscular scars. 



Hamilton group. In t!if soft shales of western New York. 



Fig. 9. An internal cast of a lai-ge perlicle-valve ; showing the impression of the rostral cavity and lai-ge 

 muscular scars, ovarian markings, vascular trunks and secondai'y sinuse.^. 

 Coi-niferous limestone. Le Roy, N. Y. 



Fig. 10. A preparation exposing the brachidium of a large individual, by the removal of the brachial valve. 

 The spiral cones have their bases parallel to the surface of the pedicle- valve and their apices 

 directed upward and inward, into the cavity of the convex brachial valve. The figure shows 

 the laterally appresseil form of the cones, the great width and anterior extension of the primary 

 lamelliE, the attachment of the latter to the crura, and tlie discrete, i-ecurved bi'anches of the 

 loop. (c.) 



Chemung grouji. Haskinsinlle, N. Y. 



Fig. 11. The cardinal portion of the bi-achial valve, enlarged ; showing the i-educed and completely divided 

 hinge-plate, and the broad dental sockets which are traversed by a crenulated median ridge. X3. 



Fig. 12. A posterior view of the cai'dinal portion of the pedicle-valve ; showing the elevation of the distant 

 teeth, the double grooving and reciirvatui-e of theii- extremities. X 3. 



Figs. 13, 14. Fragments of the spiral lamells ; showing their fimbriate outer margin. X 10. 



Fig. 15. A portion of the primary lamellse with its attachment to one of the crura. This specimen is viewed 

 from the dorsal side and shows the incurvature of the crus and its union with the outer edge 

 of the lamella. X 10. 



Fig. 16. A view of the loop drawn from the ventral side of the brachidium ; showing the thickened and 

 recurved extremities of the lateral branches. X 10. 



Fig. 17. A portion of the primary lamella and one of the crura, the latter bein^ in a pathologic condition 

 which has resulted in hypertrophy of this pai't. X 10. 

 Hamilton group. Clarke county, Indiaiia. 



