FOSSILS OF THE SILURIAN AND DEVONIAN BOOKS. 217 



Modiomorpha alta. CONRAD. 



Plate XXVI., figure 10. 



Cypricardites alta, Conrad. Geol. Surv. N. Y., Ann. Rep., p. 52 1841. 

 Modi oinorpha alia (Conrad), Hall. Prelim. Notice Lamellib. 2, p. 75 1870. 

 Modiomorplw alia (Conrad), Hall. Pal. N. Y., Vol. 5, pt. 1, pi. and exp. 1883. 

 Modwmarpha alta, Hall. Pal. N. Y., Vol. 5, pt. 1, p. 2781885. 



Shell above medium size, broad, rhomboid-ovate; length about one-third 

 greater than height ; the basal margin, for two-thirds of its length from the 

 anterior curve, is nearly straight, varying from slightly concave to nearly 

 straight, abruptly curving at the post-basal extremity, and continuing to the 

 post-cardinal margin in an oblique, gently curved line. In some examples, 

 the posterior margin is regularly curved, cardinal margin sometimes forming 

 a nearly straight line, usually gently arcuate. Anterior end produced beyond 

 the beak from one-sixth to one-fourth the length of the shell ; obliquely trun- 

 cated, obtuse, rounded below; its greatest extension is below middle of the 

 shell. 



Valves convex, gibbous on the umbonal and medial portion of the shell ; the 

 umbonal ridge is gibbous and arched upward ; the point of greatest convexity 

 is about the middle of the shell or a little posterior. The depth of both valves 

 is equal to two-thirds of the height of the shell. 



Hinge-line straight, oblique, extending for less than half the length of the 

 shell. Beaks rounded, somewhat appressed, directed forward. Umbonal 

 region not strongly defined, depressed anteriorily, becoming gibbous in the 

 middle of the shell, gradually merging into the general contour in the pos- 

 terior portion. 



Test of moderate thickness, marked by irregular concentric striae, which 

 become fasciculate and produce strong concentric ridges at irregular intervals. 

 The surface is marked by fine vascular lines, similar to those on Mod. mytil- 

 oides. The anterior muscular impression is situated close to the anterior 

 margin of r the shell, with a small retractor scar above it. Other characters 

 of the interior unknown. 



The specimen illustrated on plate 26, figure 10, is rather shorter than the 

 average. An individual of average size has the following dimensions : length, 

 twenty-three lines ; width, fifteen lines, and thickness, ten lines. 



This species has a greater proportional height than Mod. mytiloides, a 

 broader and often sub-truncate posterior end, while the anterior end is usually 

 broader and less extended ; the umbonal ridge is arcuate ; in other features it 

 is very similar. In some of its conditions this species is not easily distin- 

 guished from Mod. mytiloides. 



Formation and Locality. Pound associated with Mod. ooncentrica, mytiloides and affinis, in the 

 cherty strata topping the hydraulic limestone, in Jefferson county, Ky., and Clark county, Ind. 

 OKOL. SUR. 28 



