Shell- groiotli in Cephalopoda. 425 



rating the two structures '"' : this statement is not in the 

 Monograph; on the contrary, there will be found on p. 19 

 this description : — " Besides these two layers there is a third, 

 lining the interior of the shell. This is of very small thick- 

 ness, and consists of similar laminae to the nacreous layer, 

 &c." Tlie word " amorphous " is usually taken to mean 

 " without structure." 



Prof. Blake's descriptions are clearly inconsistent with one 

 anotlier. He did not suppose that I had made observations 

 for myself. I have done so. And I am bound to add that 

 both of his descriptions are inconsistent with the facts. We 

 must suppose that his statements of this year are intended to 

 supersede those of 1882 : let us consider them. He says, 

 ''The outcropping edges of" the fine laminee are "about 

 20,000 to the inch." He says of these lamina3, " their 

 obliquity is very slight, so that . . . they pass more than 

 one septum." It is seen by measuring the distance from 

 suture to suture in a Nautilus-shell that, to fulfil the latter 

 condition, each lamina must be from 1 to 3 inches long. It 

 is therefore obvious that, to fulfil the former condition, there 

 must be from 20,000 to 60,000 such lamina3 in the thickness 

 of the nacreous layer. And yet, as Prof. Blake correctly says, 

 " about 1000 fine laminae may be counted in its thickness." 



Prof. Blake's statement, on which he rests much of his 

 subsequent argument, that iridescence is here caused by dif- 

 fraction of light due to outcropping edges of laminga {i. e. 

 diffraction by a reflexion-grating), is based presumably on 

 the theory of Brewster ; reference to the original paper (Phil. 

 Trans. 1814, p. 397) will show that this, though the ordinary 

 reading of it, is both incorrect and incomplete. In his Mono- 

 graph Prof. Blake brushes aside the contrary conclusions of 

 Dr. W. B. Carpenter without a reference to the elaborate 

 arguments of that most accurate observer (see Brit. Assoc. 

 Eep. 1844, p. 11). I do not here commit myself to any view, 

 but examination of sections and shell-surfaces has con- 

 vinced me that the cause to which Prof. Blake ascribes the 

 observed phenomena is absolutely insufficient. For example, 

 in the most iridescent part of the shell the lines of outcrop are 

 furthest apart, and iridescent surfaces are seen between them. 

 To maintain his assertion Prof. Blake is compelled to say 

 that the septa are not iridescent. Nautilus-shells are not 

 rare ; but 1 have never yet seen one that confirms this last 

 statement. 



Let us now consider " the third layer." This was not 

 described as amorphous by Hyatt (Bull. Mus. Comp. Zool. 

 iii. p. 105, 1872) or by Blake (Brit. Foss. Ceph. p. 19, 1882). 



