SPIRIFERxE OF THE CHEMUNG GROUP. 241 



becoming quite flat in the more extended forms : mesial fold usually 

 strongly defined, rising abruptly, above the surface on either side, and 

 marked along the centre by a deep groove. The area is narrow-linear. 

 Surface marked by from ten to fourteen rounded or subangular plications, 

 those on either side of the mesial fold and sinus being usually stronger 

 than the others. When the shell is fully preserved, and sometimes in 

 partial casts, the surface is marked by strongly arching lamellose striae 

 or lines of growth, which become much crowded towards the margin 

 of the valves. 



Little is known of the interior structure of this species. A cast of the 

 ventral valve shows extended and not greasly divergent dental lamellae, 

 with a strong longitudinal median crest or septum in the muscular im- 

 pression. 



The specimens under examination are nearly all in the condition of partial casts 

 or -with the shell more or less exfoliated; while all are in single valves, adhering 

 to the sandstone or shale in which they are imbedded. 



The species is well marked and quite distinct from any other in this formation, 

 though showing a wider range of variation in forms and proportions than I had 

 originally supposed. The short forms with exti'emely extended and mucronate car- 

 dinal extremities, which are figured in the Report of the Fourth District, p. 270, 

 f. 5 & 5 a, prove, by several gradations of form, to be of the same species. This 

 variety resembles the Spirifera mucronata, for which it has no doubt sometimes 

 been mistaken. 



Among specimens from Ithaca (N.York), there are numerous small individuals 

 of this species, which, on a cursory examination, might be regarded as the young 

 of S. mucronata. The specimens are for the most part casts, and the dorsal valve 

 shows the duplication of the mesial fold; but there is no (or scarcely any) in- 

 dication of the plication in the bottom of the sinus, although there is unmistaka- 

 ble evidence of a longitudinal septum extending from the beak to near the base 

 of the muscular impression, a feature not known to exist in S. mucronata. The 

 specimens are usually semielliptical and little extended on the hinge-line, but in 

 some individuals there is a great extension of the cardinal extremities. From 

 these small individuals there is an almost insensible gradation to the larger forms, 

 as shown in the illustrations on Plate xl. In the laterally extended forms the area 

 is low, while in the shorter and more rotund forms it is more elevated, as shown 

 in the figures. The more gibbous forms have been found in a compact sandstone on 

 the Genesee river, while the smaller and attenuated ones are fronl more easterly 

 [ Paleontology IV.] 31 



