536 MR. B. B. WOODWARD ON THE GROWTH AND [June 14, 



a point situated a little way in from the posterior angle, their 

 course as they approach the margin becoming uniformly deflected 

 to the left, i. e. in the direction of the anterior angle of the aperture. 

 The nature of this arrangement is, however, more easily gathered 

 from the figure than realized from a mere description. Seen from 

 the inner aspect the relationship of the internal septum to this 

 structure becomes apparent (fig. 29). The point by the posterior 

 angle of the aperture whence the plates radiate in three directions 

 marks the junction of the septum with the outer wall ; the centre 

 of radiation a little further in falls just beside the septum in the 

 middle of its curved inner side, so that the direction of the plates in 

 the septum itself very nearly corresponds with the curvature of its 

 walls, or, to put it in another way, the ' graining ' of the septum is 

 but slightly ' on the cross.' The whole system of construction of 

 the callus, therefore, would appear to foreshadow the future 

 requirements of the animal, and its component plates to be so 

 arranged that when by erosion in the course of growth its unabsorbed 

 portions form part and parcel of the walls and septum of the shell 

 these plates shall be in the right position to impart the greatest 

 strength and durability to the whole that is possible under the 

 circumstances, for the lines of growth in the callus-formed 

 portion are of necessity lines of weakness. 



Mr. H. A. Miers was so good as to investigate a portion of the 

 crystalline layer from the callus, testing its specific gravity by means 

 of density fluids. It floated in a liquid in which aragonite and 

 tourmaline sank ; but foundered in one which would support beryl 

 and calcite. On account of its porous nature, the observed specific 

 gravity of the material must in reality be too low and the crystalline 

 callus is therefore more likely to be aragonite than calcite. 



Mr. Miers further reports : — " By taking some of the very finest 

 powder and examining it with a twelfth oil immersion, I am able 

 to find some specks which are undoubtedly calcite (by cleavage and 

 opt. char.). In section it is impossible to determine the substance — 

 because the use of the same objective with polarized light shows 

 that it consists of very minute overlapping fibres even where it 

 appears homogeneous under the quarter. Some of the powder 

 looks to me more like aragonite — little ragged fibres with straight 

 extinction, no cleavage, and the double refraction of aragonite. 



"The final result is then — an extremely fine fibrous structure ; the 

 presence of calcite proved; the presence of aragonite highly pro- 

 bable." 



The manner in which the myophore and the callus shift forward 

 with the growth of the shell in Neritina can now be readily under- 

 stood. Fresh shelly matter is added to the outer and a corresponding 

 quantity removed from the inner side. No section can well be 

 taken to prove this to demonstration in the majority of the species 

 of the genus, but in N. crepidularia it can be shown that the callus 

 does thus change its position and move through a segment of a 

 circle, keeping pace in this way with the growth of the rest of the 

 shell (fig. 30) ; and the same is true of Septaria [^=: Navicella']. In 



