287 



posterior margin. Beaks terminal, small and pointed; hinge line 

 straight, about half as long as the shell. Postero-dorsal margin gently 

 curved toward the sharply-rounded posterior extremity; ventral 

 margin gently arcuate, more strongly rounded near the beaks ; a little 

 belov7 -which the ventral surface of the shell is somewhat protruded, 

 and the margins slightly gaping, forming a distinct byssal opening. 



Surface marked by very fine, closely arranged, radiating striae, 

 which become very faint, or obsolete, on the antero-ventral portion ; 

 also by distant, somewhat indistinct, concentric undulations, parallel 

 to the margin of the shell. 



Mytilus occidentalis (n. sp.) 



Shell extremely elongate, very ventricose, the diameter through the 

 valves on the upper third of the shell as great or greater thaii the 

 breadth from the ventral to the dorsal margins ; becoming more com- 

 pressed toward the posterior, which is abruptly rounded. Dorsal line 

 nearly straight, extending about two-thirds the length of the shell. 

 Ventral margin gently arcuate to near the anterior, where it slopes 

 abruptly to the beaks. Beaks terminal, obtusely pointed ; umbonal 

 prominences subangular. 



Surface marked by closely arranged, concentric, lamellose lines, 

 parallel to the margins of the shell. No radiating stria are visible. 



This and the preceding species appear to possess all the necessary 

 characters of the genus Mytilus, as far as external characters can be 

 relied on. 



This differs from the preceding in the less prominence of the dorsal 

 line, in being destitute of radiating stria, and in the greater gibbosity 

 of the valves. 



Sub-Genus Orthoxota, Conrad. 



Orthonota ventricosa (n. sp.) 



Shell elongate quadrangular, dorsal and ventral margins subparallel, 

 or a little diverging from the anterior extremity ; length a little more 

 than twice the greatest breadth. Dorsal line very little arcuate; 

 posterior extremity broadly rounded, anterior end more narrowly 

 rounded. Beaks small, somewhat prominent and incurved, situated 

 near the anterior end. Umbonal slope ventricose, becoming more 

 depressed near the postero-ventral angle. Surface marked by concen- 

 tric undulations, which are bent upwards at, and become more distinct 

 above, the umbonal ridge. 



This shell appears to possess the characters of Conrad's genus 

 Orthoyiota ; but the beaks are more prominent, and the valves more 

 ventricose than most species of the genus yet described. The peculiar 

 plications of the dorsal line have not been observed. It greatly 



