DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 377 



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 10, Plate XXI, 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 appar- 

 ent 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 repre- 

 sented in this figure, the embryos of this species are readily 

 confounded with Meristina rectirostris, in which the triangu- 

 lar aperture is retained until maturity. The latter species is, 

 however, 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 deltaria in their different 

 phases are difficult 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 XXI, of some- 

 what 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 deltaria completed and concealed 

 by the infolding of the beak. 



Meristina Maria Hall, 1863. 



(PLATE XXI, figures 1-3.) 



Meristella Maria Hall. Trans. Albany Inst., vol. iv, p. 212, 1863. 

 Meristina Maria Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. Nat. 



Hist., p. 159, pi. 25, figs. 8-12, 1879. 

 Hall. Eleventh Ann. Kept. State Geol. Indiana, p. 299, pi. 25, 



figs. 8-12, 1882. 

 Wldtfieldia tumida* (Dalman sp.) Davidson. Supp. Brit. Sil. Brach., 



p. 107, 1882. 



This species may be regarded as presenting a general 

 external form and effect diametrically opposed to that in 



* The late Mr. Davidson identified the Waldron species with the Atrypa 

 tumida of Dalinan, the type of his genus Whitfieldia. 



