MUSEUM OF COMI'AKATIVI-; ZOOLOGY. 127 



largely animal (not mineral) matter, a substance which closely resembles 

 the matrix of the scale itself. But I know of no case of true enamel in 

 which the secretion is of such a mixed nature. 



Whether the bony material of the spine or the enamel tip begins to 

 be formed hrst I have no means of determining. I believe that the two 

 formations begin nearly simultaneously, although there is some reason 

 for thinking that the enamel is not secreted until a part of the bony 

 spine has been formed, for in a few cases I find peculiar abnormally 

 shaped tips (Plate I. Fig. 6, Plate III. Fig. 18, and Plate IV. Fig. 2G) 

 which are more readily explained upon the former assumption. These 

 all occur on individuals raised in aquaria, and I believe them to be due 

 entirely to artificial influences, — perhaps to handling. If the enamel 

 were already present over the tip of the spine, I cannot believe that its 

 point could be so distorted, whereas the matrix of the bony part of the 

 spine is probably somewhat plastic when first secreted. It is also con- 

 ceivable that, before the commencement of secretion, some distortion of 

 the papilla permanently altered its form and caused the abnormal shape 

 of the spine. This seems to me less probable, however, in view of the 

 particular forms which the points of the spines have taken. 



The material of the spine is in lamella^, though these are not as dis- 

 tinct as in the scales. The insoluble part of the point (Plate 11. Fig. 

 12, h) is doubtless made up of the first formed lamella) of the spine 

 which have separated from the rest of the cone o\\ treating with caustic 

 potash. In Figure 25 (Plate IV.) is shown the splitting between lamella) 

 caused by an oblique cut through the basal part of a spine. No sclero- 

 blastic cells, however, are enclosed between the layers, and the material 

 of the spine therefore difiers from that of the scale at its base in that 

 particular. But the absence of enclosed cells is not surprising, in view 

 of the thinness of the walls of the spine. In reaction toward acids there 

 is no diffei'ence between spine and scale. 



The number of spines upon a scale is wholly indefinite, and their 

 arrangement for the most part not subject to any precise law. They 

 are more numerous near the free (i. e. posterior) margins, and here 

 show a tendency toward an arrangement in lines parallel with the 

 edges of the scale (Plate IV. Fig. 31). I have counted over thirty 

 spines on the posterior margins of a scale whose central area was 

 nearly destitute of them. Though some spines are formed near the 

 centre of the scale, by far the greater number arise near the posterior 

 edges. With the growth of the scale, however, the margin advances, 

 leaving them farther and farther from the edge. 



