PLATK XXXIV— Continued . 



Fig-. 21. An enlargement of the external lamellose surface. 

 Hamilton group. Western Nevi York. 

 Pig. 22. An internal cast of a pedicle-valve with greatly extencieil and attenuate cardinal extremities. This 

 and approximate forms are very abundant in the coarser shales of Albany, Schoharie and 

 Otsego counties. 



Hamilton group (sandy shales). Schoharie county, N. T. 



Spirifek bimesialis, Hall. 



Fig. 23. A cardinal view of conjoined valves ; showing the lieight and slight incurvatui-e of the area. 

 Fig. 24. The exterior of a brachial valve. 



Fig. 25. The exterior of a specimen more extended on the hingo. 

 Fig. 26. An enlargement of the external surface ; showing the imbricating lamell.-E. 

 Upper Devonian. Tndependence, Iowa. 



Spiuifer mucronatds, Conrad, v;ir. posterus, var. nov. 



(This shell in one of its forms was iilentified as DeWiyris mucronata, in Geology of New York ; 

 Reporton the Fourth District, 1843 (p. 270, fig. 3), and the shell termed in that work D. acuminata. 

 Hall, is probably the same form. It is a variation from the typical form oi 8. nmcronatus, 

 similar to those represented in tigs. 15 and Ki of this plate, with broad or narrow bodies anil 

 acuminate cardinal extremities. The original term, acuminata, can not be applied to this shell 

 on account of preoccupancy.) 



Fig. 27. The exterior of a brachial valve ; showing the lamellose surface and extended cardinal extremities. 



Fig. 28. An internal cast of the pedicle-valve ; showing the impression of the muscular area but no evi- 

 dence of the median septum which exists in S. mesacostalis. 



Figs. 29, 30. Internal casts of brachial valves. 



Fig. 31. The central portion of an interior of the brachial valve, enlarged ; showing the muscular scars 

 and articulating apparatus. 



Chemung group. Toinjikin.': comity, N. Y. 



Spirifer mesacostali.'?, Hall. 



Fig. 32. View of the exterior, of an extended siiecimen ; showing the lamellose surface and biplicate 



median fold. 

 Fig. 33. A pedicle-valve ; showing the plication in the median sinus. 



Chemung group. Allegany county, N. Y. 

 Fig. 34. A cardinal view of conjoined valves ; showing the width of the cardinal areas. 



The smaller and shorter forms of this species ciiffer little in general aspect from Sp. muci-onatus in 

 some of its varieties, and in its sqiiamose surface, but the cardinal area is distinctly wider, the 

 median fold deeply duplicate and an angular iilication in the median sinus. The casts of the 

 interior are readily distinguished by the presence of a distinct septum in the ventral valve. 



