LAMELLIBRANCHIATA. 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. 



NUCUL.\ UMBONATA. 



PLATE Xr.VII, FlCiS. SI, 52. 

 Nucula umbonata. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 47, figs. .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. 



