LAMELLIBRANCHIATA. 385 



EUTHYDESMA, n. g. 



EUTHYDESMA 8UBTEXTILE. 



PLATE LXIII, FIGS. 11-16; and PLATE XCIII, PIGS. 28,29. 



Astarte mMextilis, Hall. Geol. Surv. N. Y., Rep. Fourth Dist., p. 245, fig. 6. 1843. 

 Cardioinnrpha suhlextUU, Hall. Prelim. Notice Lamellibranchiata, 2, p. 93. 1870. 



" " " (textUis, in eirtrr). Pal. N. Y., vol. v, pt. 1. Plates and Explanations 



PI. 63, figs. 11-15. 1883. 



" undulata, " Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 63, fig. 16. 1883. 



Shell large ; body broeidly sub-ovate, with a sub-alate cardinal expansion ; 

 length one-fifth greater than the height; basal margin very regularly curv- 

 ing to the post-inferior extremity, which is sub-truncate above. Cardinal 

 line rigidly straight. Anterior end short and regularly rounded. 



Valves moderately convex in the posterior part, very gibbous in the mid- 

 dle and umbonal region. 



Beaks sub-anterior, large, prominent and incurved. Umbonal slope rounded 

 and merging into the general convexity of the shell. 



Surface marked by fine, sharp, elevated concentric striaa, which, in well- 

 preserved shells, are crossed by fine, slender, radiating striae, giving the sur- 

 face a cancellated appearance. At the intersection of the striae, minute 

 nodes are formed. In old shells the surface becomes raised into unequal 

 concentric undulations and also into stronger undulations, which cross the 

 lines of growth obliquely on the middle of the shell. Some specimens show 

 two low plications along the post-cardinal slope. 



Hinge-line straight and marked by a distinct, continuous ligamental 

 groove. Other characters of the interior unknown. 



Two specimens measure respectively 37 and 45 mm. in length, and 80 and 

 37 mm. in height. The largest specimen observed has a length of about 

 75 mm. 



A careful comparison of this fossil with other species shows it to be so 

 entirely distinct in its form and exterior characters, as well as in the features 

 of the hinge, that there are no allies in our geological formations with which it 



