FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 137 



Trematospira helena. . SP. 



Plate XXXII., figures 40 to 43. 



Shell very small, broadly ovate or sub-globose, very gibbous ; somewhat 

 longer than wide ; greatest width below the middle of the length, but not as 

 near the base as shown in the figures 40 and 41. Surface plicated. 



Ventral valve regularly curved from beak to base ; greatest convexity at the 

 middle of the valve, from where it slopes in a regular curve to lateral and basal 

 margins, but rather abruptly to cardinal lines ; beak in proportion to the size 

 of shell prominent, elevated and incurved over that of the opposite valve; 

 the place of the apex is occupied by a round, plainly visible foramen ; lateral 

 and basal margins form a regular curve, with a slight emargination in the 

 central portion of the base, which is not shown in figures 40 and 41 ; from the 

 umbo to the front a mesial depression is marked. 



Dorsal valve equal in convexity to the other ; most prominent at the mid- 

 dle, and curving regularly to side and front margins, but more rapidly to the 

 cardinal line. This valve has also a mesial depression indicated ; its umbo is 

 prominent, and its beak strongly incurved into the opposite valve below the 

 foramen of the same. 



Surface is marked, in proportion to the size of the shell, by very strong, 

 rounded plications, all of which, with the exception of those in the mesial 

 depressions, reach to the beak ; the ventral valve has five and the dorsal valve 

 four on each side of the sinus. The sinus of the ventral valve has in its bot- 

 tom a greatly smaller plication, which starts at the lower side of the umbo, 

 and bifurcates at about the middle of its length ; in some shells the mesial rib 

 does not dichotomize. The plication in the mesial depression of the dorsal 

 valve is also somewhat smaller than tfre others, but the difference in size is not 

 so marked as in the ventral valve, and consequently the mesial sinus of the 

 dorsal valve is not as conspicuous as that of the ventral. This mesial rib of 

 the dorsal valve starts at the beak, and bifurcates below the umbo ; both prongs 

 attain at the base almost the same strength as the adjacent ribs, thus leaving 

 in a front view of the shell only a faint indication of the dorsal sinus. All 

 these radiating plications are crossed by fine, closely set, lamellose concentric 

 lines of growth, which give the surface a somewhat rugose appearance. The 

 figures, which are very defective in not representing important features of this 

 beautiful little shell, are enlarged to about double size. 



Formation and Locality. Found in the lower strata of the Niagara group (which by some geolo- 

 gists are placed in the Clinton group), in the quarries east of the city of Louisville, Ky. It is exceedingly 

 rare, only three, but very fine specimens, are in my collection, while I have never seen it in any other 

 cabinet. This shell has a close resemblance to Trem. globosa, Hall, from the Lower Helderberg group, 

 GKOL. SUR. 18 



