THE I'OKMATION OK THE ORGANS. 61 



curve and appears more arched (Figs. 27 B and 18, p. 36), and finally the 

 umbones appear. The form of the shell seems to be developed in this way in 

 many Lamellibranchs, as may be seen from the figures given by Lovkn and 

 Jackson and other authors. Jackson (No. 22) gives a special description of 

 its development in Ostrea (Fig. 27 -I-O, and we were ourselves able to observe 

 its rise in a very similar manner in Dreissensia (No. 27). 



It is hardly necessaiy for us to point out the great difference that exists in 

 the two forms just named between the larval and the adult shell, and this is 

 still more the case in such Lamellibranchs as Pecten and the Avicnlidae, the 

 larval shell of which resembles that depicted in Fig. 27 C. This latter form 

 of shell (the prodissoconch of Jackson) * represents a stage passed through by 

 many Lamellibranchs. Jackson is therefore inclined to consider this form 

 of shell as specially primitive. In keeping with this view, we find that NuctUa, 

 which, in consequence of other features may be regarded as a primitive form, 

 and which is found in the lower Silurian [Ctenodonta, one of the Nuculidae, 

 occurs in the Cambrian], has a shell of somewhat the same shape as the 

 above. 



The young shell grows by the secretion from the mantle-cells of 

 new calcareous material ; this is deposited both on its inner surface 

 and at its margin. These deposits give rise to the lamellate and 

 zoned character of the adult shell. In older stages, these growth- 

 processes take place chiefly at the peripheral parts of the mantle. 

 There appears to be no essential difference between the manner of 

 formation of the inner (nacreous) and the outer (prismatic) layers of 

 the shell ; the one may pass into the other. The prismatic structure 

 of the outer layer is apparently due to the fact that the originally 

 rounded calcareous granules became polygonal through mutual 

 pressure. For details as to the formation of the Lamellibranch shell 

 we are indebted especially to the researches of Tullberg (No. 56), 

 Ehrenbaum and F. Muller (Nos. 1 1 and 38). 



The growth of the periostracum also takes place at the edge where 

 this latter passes over into a fine cuticle which covers the (ectodermal) 

 epithelium of the pallial margin. At the periphery, where the cal- 

 careous shell ends, we thus have the same condition as is shown in 

 early embryonic times by the whole shell ; the shell-integument, as 

 a cuticle, covers the mantle-epithelium. Indeed, the whole of the 

 Lamellibranch shell is to be regarded as a cuticular structure covering 

 the mantle-epithelium. 



* In contradistinction to the adult shell (the " dissoconch " ) Jackson has 

 named the young arched form with well-developed umbones (Fig. 27 C) the 

 " prodissoconch," because it usually precedes the adult form of shell, as has 

 been shown above. 



