74 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.51. 



projecting lines, as well as by delicate longitudinal and transverse striae. Area 

 triangular ( (d) ), fissure ( (e) ). Specimens vary much in size 



The omissions marked b,y dotted lines above, are parts of the de- 

 scription which do not apply to S. trescoiti. The differentiating char- 

 acters are therefore found in the following particulars: {a) The hinge 

 line does not quite reach the full width of the shell in our specimens. 

 (&) The cardinal angles are rounded or slightly angular, {c) The 

 plications on the brachial valve vary with size of the shell, addi- 

 tional plications making their appearance at the cardinal angles with 

 development of the shell. The smallest specimen in evidence (see 

 pi. 1, figs. 2 and 4) is 5 millimeters wide and shows one plication 

 each side the median fold of nearly the size of the fold, and a second 

 one faintly expressed outside the first. The cardinal extremities are 

 broadly rounded, presenting the form of Spirifer crispus, var. sim- 

 plex Hall. A specimen about 7 mm. wide (H. S. W. Cat. No. 1429.2) 

 shows three plications each side the fold and the fold is distinctly 

 broader than the first plication ; the extremities are rounded. Speci- 

 mens 8 or 9 mm. wide have the extremities slightly angular and four 

 plications (and in one specimen a faint fifth) are seen each side the 

 fold. Larger specimens, up to the largest seen (17 mm. wide), show 

 only four well defined and faintly a fifth plication each side the fold. 

 The fold in the smaller as in the larger specimens shows the 

 " groove along the middle " described by Davidson as characteristic of 

 tS. elevata Dalman. The pedicle valve shows but four well-defined 

 plications each side the sinus which, at the front of the larger speci- 

 mens, is quite broad, equaling the distance from first to third plica- 

 tion. The small pedicle vah'es differ from S. crispus in the high, 

 strong, and overarching beak, and a specimen 6 mm. wide, a mold of 

 the interior of a pedicle valve, shows a distinct septum as in adult 

 specimens, (d) The triangular area of the pedicle valve is not broad 

 but is undefined, as shown by AVhitfield in figures of Spirifer crispus 

 and S. vanuxemi^, and slopes gradually off into the sides of the shell 

 without any sharp angle defining its limits. Specimens showing this 

 character are molds of the interior, and it is therefore possible that the 

 limits are more sharply defined exteriorly as expressed by Davidson's 

 figures, (e) The fissure (delthyrium) is narrow and high (see pi. 1, 

 fig. 20), and no case of a pair of deltidial plates has been discovered 

 on our specimens. Davidson mentions them in his description, but 

 none of his figures show them. 



The dental plates of the pedicle valve are thin and elongate. In 

 small as well as large specimens there is a thin, deep median septum 

 reaching halfway from beak to front. The surface is covered by 



iBrit. Sil. Brae, p. 95. 



2 Pal. New York, vol. 8, pt. 2, pi. r?6, figs. 2 and 11. 



