Shell-growth in Cephalopoda. 423 



So far as Sepia is concenied, Prof. Blake tacitly admits not 

 only the originality but the correctness of the observations 

 made by Riefstahl and myself. Where I ditfer from Rief- 

 stahl as to the facts, and in the inferences based on those 

 facts with regard to Sepia, he also gives me his support. 

 This support is valuable, for Prof. Blake knew all that we 

 have discovered about the hard parts six years ago. All 

 students will regret that his observations were never pub- 

 lished. Prof. Blake apparently accepts the view that successive 

 chitinous membranes are given otF by the body-surface and 

 subsequently calcified (a view which I claimed to defend rather 

 than originate), and he joins me in ascribing to this process 

 the formation of nacreous layer and septum. This view 

 differs from that advocated in Blake's Brit. Foss. Ceph. 

 p. 19, lines 23-27; it gives me pleasure to suppose that 

 Prof. Blake's change of opinion is partly due to my new 

 facts and arguments. 



Prof. Blake denies " that in a Nautilus the earlier septa are 

 approximate^ the middle ones far apart, and the later ones 

 approximate again." It is hard to see how this meaning can be 

 extracted from my sentence, viz. " In the Nautiloidea the septa 

 are still \i. e. at the present day] far apart^ but approach in 

 old age " ; and I have repeatedly verified the remarks on 

 p. 30 of his Monograph. Although he there says nothing as 

 to the relations of the septa in the young uncompleted shell, 

 he need not suppose that I thought his observations " too 

 partial to be of value " ; there was simply no occasion to 

 allude to them. 



I proceeded to say that the Ammonoidea soon differed from 

 the forms with approximate septa which Hyatt, Foord, and 

 others regard as archaic : — '^ So early as the Goniatites the 

 septa are far apart in proportion to the diameter of the whorl." 

 Prof. Blake (who seems to place all Goniatites in one genus) 

 reminds me that G. Sagittarius of the Devonian has very 

 close-set septa, and asks if I can then maintain my state- 

 ment. Certainly ! I did not say " in all Goniatites " or even 

 " in most Goniatites." The septa in one species may be ever 

 so crowded ; this does not affect the septation in other species, 

 in other genera, in other subfamilies. Prof. Blake cannot be 

 guilty of so obvious a fallacy in logic ; he merely misunder- 

 stood the statement. 



Finally, Prof. Blake approves the suggestion to divide the 

 Cephalopoda into three orders, dropping the old terms Tetra- 

 branchiata and Dibranchiata. 



These orders are: — (i.) Nautiloidea, Cephalopoda in which 

 the protoconch is not preserved, although coiling takes place: 



29* 



