72 MEMOIRS OF THE STATE MUSEUM 



Under the discussion of that species, reference has been made to the impos- 

 sibiHty of separating these two species, in their earlier stages, and we are 

 left to regard the impression of the specific characters as of subsequent 

 development. 



DEVELOPMENTAL VARIATIONS 



The surface characters being unvariable, the important changes in 

 development are confined, as far as observable, to the pedicle-aperture and 

 deltidial plates. As already observed, the beak is incurved so early in the 

 history of the individual that these embryological changes can be observed 

 only in very young specimens. This incurvature of the ventral beak 

 appears to become fixed earlier in the normal than in the elongate form. 

 For example, figure lo, plate 7, represents an elongate individual with a 

 length of 2.5 mm. and a width of 1.5 mm., with an open triangular foramen, 

 and no apparent development of the deltidial plates, but the normal form of 

 the same size has the plates developed, the foramen nearly circular and the 

 beak incurved. In the condition represented in this figure, the embryos of 

 this species are readily confounded with Meristella rectirostra, in which the 

 triangular aperture is retained until maturity. The latter species is, how- 

 ever, distinguishable in all the later stages of its existence by the body of 

 the shell being broader and the ventral beak narrower and more attenuate. 



Individuals which show the deltidium in its different phases are dififi- 

 cult to obtain on account of the tendency of the beak to incurvature as 

 soon as the plates begin to form. An individual is represented in figure 7, 

 plate 7, of somewhat abnormal height of beak, showing an intermediate 

 stage of growth in the plates and the formation of the foramen ; and in 

 figure 8, an individual of the same size with the foramen circular, and the 

 deltidium completed and concealed by the infolding of the beak. 



