Mr. Jeffreys on British Mollusca. 189 



and by degrees they are elongated, until their length is much 

 greater than their breadth ; they also expand laterally to a certain 

 extent after their first formation; but this growth ceases, so that 

 the rootlet has a fixed diameter. The cell-division seems to be 

 repeated in these cells in the direction of their length after they 

 have attained their full diameter. While young, near the tip of 

 the root, they are densely filled with protoplasmic substances ; 

 as they expand they appear clearer, and contain only a moderate 

 quantity of protoplasm, with abundant watery cell-sap. The 

 rudimentary cells developed in the very centre of the point of 

 growth become cells of much less diameter and more elongated 

 form, and constitute the rudiment of a fibro- vascular cord run- 

 ning through the centre of the rootlet ; at a little distance from 

 the point, traces of spiral markings may be detected on the walls 

 of some of these cells, which are becoming vessels, — the distin- 

 guishing marks of the fibro-vascular bundles. Higher up in the 

 root, the central fibro-vascular cord is clearly recognizable, sur- 

 rounded by parenchymatous cells, themselves enclosed by a 

 continuous layer of delicate epidermal cells. In these roots the 

 epidermal cells do not grow out in hairs (radical fibrils). 



" Roots of this kind show very clearly that the elongation of 

 roots takes place by increase at the point only. This is seen by 

 noticing the relative dimensions of the cells in the different parts ; 

 but it may be proved still more evidently by marking the roots, 

 when of some length, at equal distances, with touches of Indian 

 ink. When we watch the further growth of a root thus marked, 

 we see that the spots on the upper part of the root do not be- 

 come removed to a greater distance from each other, but new struc- 

 ture is added on below the marked parts. The same important 

 law of growth is illustrated by the natural marks made by branches 

 arising from the roots, which remain permanently at their ori- 

 ginal distance apart, as may be clearly seen in the transverse 

 streaks on the surface of the root of a carrot." 



XX. — Additional Gleanings in British Conchology. 

 By J. GwYN Jeffreys, Esq., F.R.S. 



In continuation of my notices on this subject, I have only occa- 

 sion to make a preliminary remark, that, although I have at 

 present no new species to describe, the communication of any 

 facts which may serve to increase our knowledge of already 

 known species is not less valuable or interesting than the publi- 

 cation of novelties. 



Acephala Lamellibranchiata. 

 Teredo megotara, Forbes and Hani. Brit. Moll. vol. i. p. 77. 



