LAMELLIBRANCHIA TA. 
321 
angular, extending from the beak to the post-basal extremity, declining 
abruptly to the cardinal line. 
Surface marked by strong, regular, lamellose, concentric striae which 
appear to be made up of aggregations of extremely fine striae. 
Six or eight strong transverse teeth are preserved on each side of the beak. 
Three specimens measure respectively 9, 8 and 7 mm. in length, and 4.5, 
4, and 3.5 mm. in height. 
This species is remarkable for its sub-central beaks and obliquely truncate 
posterior end, in which characters it differs from all others of the genus 
described in this volume. In its surface characters it is similar to N. lirata 
except that no radiating striae have been observed, but the form of the shell 
is otherwise very different. 
Formations and localities. In the shales of the Hamilton group, on the shore 
of Cayuga lake, and in the lower part of the Chemung group, at Ithaca, N. Y. 
Nucula umbonata. 
PLATE XLVII, FIGS. 51, 52. 
Nucula umbonata, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 47, tig-s. 51, 52. 1883. 
Shell large, broadly ovate, wider in front; length somewhat greater than the 
height; basal margin gently curving in the middle and abruptly rounded at 
both extremities. Cardinal line sloping both anteriorly and posteriorly from 
the beaks. Anterior end very short and rounded. 
Valves gibbous, rising from the basal margin to the umbo, which is the 
point of greatest gibbosity; sloping abruptly to the anterior and posterior 
cardinal margins. 
Beaks at about the anterior fourth of the length, obtuse, scarcely rising 
above the hinge-line. Umbo very prominent and gibbous. Umbonal slope 
defined, rounded, extending from the beaks to the post-basal extremity. 
Test and markings not preserved in the specimen described, which is a 
partial cast of the interior, showing the elongate vertical anterior muscular 
scars. 
