PLATE L. 



(Figures 1-5 copies; 6, 8, I2-U, 28-31, 41-48. 51, 63 by E. Emmons; 7, ii-U, IR, 17, 20. 23. 24, 32-35 by G. B. Simpson; 

 15, 1«, 19 by J M. Clarke; 21 by F. 1$. Meek; 23, 25, 36-4U, 4!), 60 by R. P. Whitfield ) 



Legend ; t. Teeth. b. Dental sockets. .'f Median eeptum (pedicle-valve), 



d. Dental [dates. c. Crura. a'. Median septum (brachial valve), 



hp. Hinge-plate. 



Genus RETZIA, King. 



I'.ige lOi. 



Retzia Adrikni, do Venieiiil. 



Figs. 1, 2. Doi'sal and ventral views of a typical sjiecimen ; showing ihe characters of the exterior. 



Fig. 3. A doi-sal view of a smaller example, from which a portion cjf the brachial valve has been removed, 



exposing the median septum and a portion of one of the si>iral C(mes. 

 Fig. 4. An enlarged view of a shell which has Ijeen ti-ansversely sectioned just in front of the nmbones; 



showing the hinge-plate, dental sockets and median septum. 

 Fig. 5. 'J'he interior of a portion of the bi-achial v.alve ; showing Ihe hinge-plate and median septum. 



The above figures are cojiied from QiuLERT, Annates Sci. Geol., t. xix, No. 1. Ib86. 

 Fig. (i. A dorsal view of a somewhat flattened example. 

 Fig. 7. A more rotund individual, imperfect about the ante-lateral margin. 

 Fig. 8. A view of a similar specimen of larger size. 

 Fig. 9. A dorsal view of the umbonal region of conjoined valves; showing the excavate cardinal slopes 



of the pedicle-valve ami the concave coalesced deltidial plates. X 2. 

 Lower Devonian. DeparUiiient cle la iSa)'the, France. 



Genus PTYCHOSPIRA, gen. nov. 



Page 112. 



Ptychospira feeita, von Buch. 



Fig. 10. A profile view of an average individual ; showing the coarse plication, and the exten.sion of the 



anterior margin. X 2. 

 Fig. 11. A dorsal view of the same specimen ; showing the coalesced deltidial plates. X 2. 

 Middle Devonian. Eifel, Germany 



PxYCHOsriRA LONGiuo.STRis, Kaj.ser. 



Fig. 12. A dorsal view of a specimen ; showing the narrow and elevated beak of the pedicle-valve. 

 Middle Devonian. Eifel, Germany. 



PxYCHOsriRA (cf.) sexi'Licata, White iiiid Whitfield. 



Figs. 13, 14. Dorsal and ventral views of a specimen, probably referable to this species. 

 Burlington limestone. Biirthigton, lima. 



Subgenus HOM(Ji:OSPIRA, s.-gkn. nov. 



Pase 112. 

 HOMCEOSPIRA EVAX, Htlii. 



Fig. 15. A dorsal view of the youngest shell observed. In this stage of growth the beak of the pedicle- 

 valve is ei-ect and the deltidial plates not developed ; the surface of the valves is smooth and 

 evidence of plications is visible only outside of the second concen)i-ic growth-line. X 25. 



Figs. 16, 17. Profile and dorsal views of <a full-grown specimen ; showing the adult characters of the exterior. 



Fig. 18. The umbonal region of a young shell in which the deltidial plates have united at the base, enclos- 

 ing an oval foramen. X 7. 



Fig. 19. A similar view of a more fully grown individual with unusually erect beak and nearly ovate 

 foramen. X 3. 



Fig. 20. The interioi' umbonal portion of a fullyinatured example ; showing Ihe complete obsolescence of 

 the deltidial plates and the enclosure of the foramen by the sulistance of the valve ; also the 

 projecting, recurved teeth. X 2. 



Niagai'a group. Waldron, Indiana. 



Genus RHYNCHOSPIRA, Hall. 



Page los. 

 RhYNCHOSPIRA FORMOSA, Httll. 



Fig. 21. A dors.il view of a typical specimen ; showing the character of the exterior. 



Fig. 22. The inteiior of the cardinal portion of the peilicle-valve of a normal ailult; showing the circular 

 foramen, the completely coalesced deltidial plates, and the teeth. X 2. 



Fig. 23. The interior of a pedicle-valve ; showing similar characters with less detail. 



Pig. 24. The interior of a brachial valve ; showing the form of the hinge-plate, the dental sockets and the 

 median septum. 



Fig. 25. An interior view of the cardinal portion of articulated valves ; showing the elevation of the hinge- 

 plate, and the median septum of the brachial valve, and the muscular depi-ession of the 

 pedicle-valve. X 2. 



Lower Helderberg group (Shaly limestone). The Hehlei'berys, N. Y. 



