PLATE XXXIV— Continued . 



Pig. 31. An enlargement of the external lamellose surface. 

 Hamilton ffroup. Western New York. 

 Fig. 22. An internal cast of a pedicle-valve with greatly extended 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. 



Spirifer bimesialis, Hall. 



Fig. 23. A cardinal view of conjoined valves ; showing the height and slight incurvature of the area. 

 Fig. 24. The exterior of a brachial valve. 



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

 Fig. 26. An enlargement of the external surface ; showing the imbricating lamellce. 

 Dpper Devonian. Independence, Iowa. 



Spirifer MuCRONAras, Conrad, var. posterus, vur. uov. 



(This shell in one of its forma was identified as Delthyris mneronata, in Geology of New York ; 

 Report on the Fourth District, 1843 (p. 270, fig. 3), and the shell termed in that work/). ocumiTiata, 

 Hall, is probably the same form. It is a variation from the typical form of (S. mueroTiatns, 

 similar to those represented in figs. 15 and 16 of this plate, with broad or narrow bodies and 

 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 iS'. mesacostalis. 



Figs. 29, 30. Internal casts of bi-achial valves. 



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

 an<l articulating apparatus. 



Chemung group. TompHng county, N. Y. 



Spirifer mesacostalis, Hall. 



Fig. 32. View of the exterior, of an extended specimen ; showing the lamellose sui-face and bipiicate 



median fold. 

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



Chemung gi-oup. Allegany county, N. Y. 

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



The smaller and shorter forms of this species differ little in general aspect from Sp. mucro^tatiis m 

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

 median fold deeply duplicate and an angular {ilication in the median sinus. The casta of the 

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



