434 



NATURAL SCIENCE. 



Dec, 



by a series of hollow cones at irregular intervals. It is possible that 

 the actual moment of casting off the slough was after the emission of 

 the generative products, when the visceral cone was flaccid ; this 

 explanation coincides with Seeley's explanation of the origin of septa- 

 tion itself, but it is not exposed to the objections brought against the 

 latter (14), There is, of course, no reason why " sheaths " should not 

 have been formed in Nanno ; but, to judge from the descriptions and 



figures of Holm and Clarke, no trace 

 of them has yet been seen. 



The feature of chief interest in 

 Nanno is the apical swelling of the 

 siphuncular passage. This was 

 considered by Holm to represent the 

 initial chamber, " Anfangskammer," 

 by which he presumably meant the 

 protoconch. Such, indeed, in spite 

 of its large size, it appears to have 

 been, and as Clarke observes, it 

 probably contained the body of the 

 young animal for a considerable 

 period. That the forward move- 

 ment of the young was quite gradual, 

 is possibly indicated by the soHdity 

 of the calcareous deposit lining the 

 walls of this apical chamber. If 

 this suggestion be correct, it be- 

 comes interesting to compare this 

 protoconch with the structures 

 noticed above in the so-called 

 Orthoceras from St. Cassian (see 

 p. 425). The depression seen so 

 clearly in Fig. 4, b, may well re- 

 present the constriction and the 

 " shoulder " of the cicatrix alluded 

 to by Hyatt in those specimens ; 

 while the plug of calcite in the 

 Orthoceras is the diminished repre- 

 sentative of the calcareous lining in 

 Nanno. There is perhaps no impor- 

 tance to be attached to the fact that a swelhng of the shell at a little 

 distance from the apex occurs in Nanno aulema, Orthoceras politum, 

 Bactrites, and Agoniatites fecundus ; but it is certainly more worthy of 

 remark than any resemblance to Gomphoceras and Oncoceras, such as 

 Clarke insists upon. 



Assuming that the somewhat similar structures seen in these 

 forms are protoconchs in the strict sense, then it is clear that there is 

 a great difference between them and such other primitive forms as 



Fig. 6. — Actinoceras. i. Front view of 

 a weathered apical extremity, from 

 which the shell has been removed. 

 2. Side view of another specimen. 

 f, a large aperture at the apex ; 

 /', the openings of small passages 

 leading through the calcite in- 

 filling the neck-tube to the si- 

 phuncle, and probably representing 

 the last relics of the attachment 

 of the siphuncle to the wall of the 

 neck-tube. 



(Both figures natural size ; drawn by 



A. H. Foord ; the block kindly lent 



by Dr. H. Woodward, F.R.S.) 



