16 MEMOIRS OF THE STATE MUSEUM 



vexity. In the next stage, the depth of the ventral valve has noticeably 

 increased over that of the dorsal, and, as in the latter valve the median sinus 

 has become distinctly developed, the difference in this respect becomes 

 emphasized. The divergence of the valves in convexity becomes increased 

 until maturity, and this growth is accompanied in the ventral valve by a 

 correspondingly increasing incurvature of the beak. 



Beaks. In the incipientshell, the beaks are erect and distant, but not 

 prominent. By the development of the broad sinus on the dorsal valve, the 

 beak of this valve becomes relatively less prominent and apparently more 

 closely appressed to the cardinal line. On the opposite valve, every increase 

 in convexity is accompanied by a corresponding increase in the incurvature 

 of the beak, and as the "shell approaches maturity, the incurvature becomes 

 so great, that it has been necessary, in the drawings which are here given 

 showing the features of the cardinal area, to represent the beak as broken 

 away. 



Foramen. The earliest stages of growth show a remarkable feature in 

 the triangular, marginate, subequal fissures on the valves. This character 

 may prove of a high taxonomic value, as it clearly indicates the relation- 

 ship, in this respect, of Orthis to the Linguloids, and places the genus near 

 the inarticulata. The foramen upon the ventral valve is, in every stage 

 of development, open and free for the protrusion of the pedicle. Del- 

 tidial plates are absent in every stage of growth. In a secondary stage, a 

 cardinal process begins to form in the apex of the dorsal foramen, soon 

 widening and becoming tripartite. As age increases, this process is projected 

 into the ventral foramen, never quite filling it, always leaving room for the 

 protrusion of the pedicle. In immature conditions the cardinal process 

 is attached to the shell only at the apex of the foramen, but with ma- 

 turity it comes in contact with the sides of the foramen, and at this 

 stage entirely fills the dorsal aperture. With the increasing incurvature 

 of the ventral beak and cardinal area, the a*pertures of the two valves 



