Report of the State Geologist. 231 



The variability of the transverse plate between the dental lamellae 

 in the ventral valve is very great, as was shown by Prof. Hall, as 

 early as 1867.* In certain Silurian and Devonian species of Spirifer , 

 the apical portion between the dental lamellae is gradually filled up, 

 in Devonian species such as "S. macronota and S. medialis. Hall, there 

 is a thickened transverse septum [plate] extending from one-third to 

 one-half the length of the fissure from the apex, and this feature is 

 quite independent of the pseudo-deltidiam" (Hall, ibid., p. 254). In 

 S. alta, Hall, from the Chemung, this transverse plate is well developed 

 and has upon its lower side " a narrow semicylindrical depression, 

 extending to the apex " (ibid., p. 252). It will be seen that as yet we 

 have not a fully developed " canalifei^ous plate," nor the pointed, 

 inwardly directed extension of this plate. This is first developed in 

 Ohio specimens of S. carteri, as found in the Bedford shale, associated 

 with a fauna that has a Hamilton group expression. This is the first 

 species, geologically, known to have the split tube and a punctate 

 shell combined. In the Burlington limestone, however, we have 

 Spirifer plenus, Hall, which has a decidedly punctate shell structure, 

 and also the characteristic "twilled cloth" like surface ornamenta- 

 tion. This species has a well developed transverse plate, but not the 

 faintest indication of a depression upon its lower side. This shows' 

 that we should not place too much classificatory value upon a punctate 

 structure for species of this genus. In S. herricki, n. sp., this tube 

 has become solid and attached to the bottom of the valve, while in an 

 undescribed species from the St. Louis group, two-thirds of the 

 umbonal cavity has become filled up, terminating in a short, thick 

 median septum, above which and originating from the upper portion 

 of this filling is the short split tube. * 



This canaliferous transverse plate appears to have served for the 

 attachment of the pedicle muscles. Prof. King says f : " Can it 

 have afforded attachment to the capsular [the inner end of the 

 pedicle] muscle ? In this case it might be concluded that the canal 

 itself has served as a muscular fulcrum for the dor aal pedicle muscles, 

 which ordinarily have one exti'emity implanted on the intercrural 

 plate of the dorsal valve, and the other attached to the pedicle." 



The deltidial covering is regularly convex and smooth for about 

 one-half its length from the apex, then becoming thinner and lamel- 

 lose, leaving a semicircular opening near the area of dorsal valve. I 

 have seen this covering in S. carteri from the Bedford shales of Ohio 

 and the soft shales of the Choteau group of Missouri; in S. texta from 



* Palaeontology, New York, vol. iv, p. 251. 



t Annals and Magazine of Natural History, 4 ser., vol. 2, p. 22. 



