704 RECORD OP CURRENT RESEARCHES RELATING TO 



vigour to the defonco of tlio position which he took up a number of 

 years ago, as to the difference in the characters of the coil of the shell 

 in Spirula on the one hand, and Nautilus, with the Ammonites, on the 

 other ; the latter are regarded as revolutely spiral or coiled over the 

 back of the animal, whereas those of Spinda are involute. 



It has long and well been known that the " fossil shells of Cephalo- 

 pods exhibit a progressive uncoiling from Nautilus to Ort]ioceras,fiiid. from 

 Ammonites to Baculites" but, as Professor Owen well remarks, anato- 

 mical evidence was required to demonstrate that there had been coiling 

 in the reverse direction ; that has been suj)plied by his dissections of 

 Spirula and of Nautilus, and the author now demonstrates that it is 

 not always necessary to have seen an animal before we are able, with 

 proper insight, to tell a very great deal about its organization. Apply- 

 ing this to the Ammonite, we find evidence in suj)port of Owen's 

 long-held view as to the shells of these animals having been external ; 

 while specimens in which injury and repair have been effected j)oint 

 to the mantle of these forms having had just the same structure as in 

 the living Nautilus, and as having been ajjplied to the shell, and not 

 to have been " muscular and inapplicable to the last chamber as in 

 Spirula." Another point of similarity between the first two of these 

 forms is that the shell is porcellano-nacreous in structure, and not 

 simply nacreous as in Spirula. Between Spirula and Ammonites there 

 is, however, a very remarkable similarity ; in both, the siphons in the 

 shell are ventral, or ventro-marginal, but in relation to the curves of 

 the shell, the siphon of Spirula is " internal " or " ento-marginal," 

 whereas in the Ammonitidfe it is " external " or " ecto-marginal." To 

 turn to the Nautilidfe, where the siphon is central or subcentral in 

 most cases ; in others (rare) it is ecto-marginal, and still more rarely 

 it is ento-marginal. It is here of interest to observe that it may 

 change its position as it passes through the different chambers of the 

 shell, so that the position, which is a sign of immaturity in the 

 existing Nautilus, was longer retained in the old Tertiary species. 



Professor Owen deals also with the history of those curious 

 structures, which under the names of Trigonellites, or Aptyclius, have 

 been referi'ed to the Cirripeds, or have been regarded as j^arts of the 

 Ammonite's gizzard, or as protective plates of the nidamental glands 

 in the female ; a careful discussion of the question leads the author 

 to accept and support the view held by Van der Hoeven, that the 

 shells in question formed two shelly supports for the hood of Am- 

 monites. 



Turning to the chambers of the shell. Professor Owen next describes 

 what obtains in Vermetus gigas and in Spondylus varius, as well as 

 what is seen in Spirula and Nautilus. As to the " final cause " or 

 function of the chambers, the author holds to his original opinion that 

 they enable the Nautilus to rise, and the Spinda to sink. As to the 

 modes by which the shell may become complicated, it will be of 

 interest to note that the septa may be (1) simple and distinct, attached 

 only by their circumference, or (2) attached subcentrally to each other 

 as well as by their circumference to the shell-wall ; or (3) attached 

 marginally to the shell-wall and having an organized vascular mem- 



