78 MEMOIRS OF THE STATE MUSEUM 



2 mm. in length. The differences appear to be so slight that the characters 

 of the larger may well be applied to the smaller individual. 



The shell is nearly circular, flattened, and with the beaks not incurved, 

 but directed outward. The area of the ventral valve is broad, triangular, 

 open, and extends nearly the entire length of the cardinal line. The incip- 

 ient dorsal fold and ventral sinus extend nearly to the beaks, and on each 

 side there are about ten radiating striae. Radii are also present on the fold 

 and in the sinus. 



DEVELOPMENTAL CHANGES 



The changes in the shell from advancing growth are principally the 

 gradual widening of the valves, on account of the extension of the cardinal 

 line and extremities, and the incurving of the beaks, from the progressive 

 increase in the depth of the valves. From being circular in outline, the 

 shell slowly widens until it is one-seventh wider than long. The ventral 

 beak in old specimens is so arched over the area as to nearly conceal it, and 

 prevent the opening of the valves to any extent. In the early stages, the 

 depth of the conjoined valves is about half the length of the shell, while 

 in obese mature forms, the depth is equal to the length. 



The deltidial plates first appear as narrow elevated laminae along the 

 sides of the fissure under the ventral beak. A specimen about half-grown 

 shows them as represented in plate 6, figure 9, consisting of triangular 

 plates approximately as in figure 3', of the following diagram. They are 

 'subsequently united along their inner margins, and rarely, in the material 

 at hand, can any appearance of a foramen be discovered. In old shells, 

 the growth and thickening of the deltidium makes it rugose, and it nearly 

 closes the area. 



From an examination of a number of -species of Spirifer showing 

 considerable variety in the mode of development of the deltidium, we 

 believe that there is no esssential difference, and that all intermediate con- 

 ditions occur in this group, between the features represented in Spiriferina 



