378 Reviews — Prof. Dr. G. Lindstrom — 



it is very constant in any given species. 1 The septa are continuous 

 from the ventral (convex side) to the dorsal side, as may be seen 

 in many of the casts, 3 where the sutures continue uninterruptedly 

 across; in the interior of the shell, however, this continuity is 

 broken. Here only the first sigmoid septum is entire, all the 

 succeeding ones having a broad, elongate, elliptical empty space 

 or lacuna, running longitudinally and slightly expanding anteriorly, 

 in accordance with the increasing diameter of the shell (Figs, c, d). 

 The organic deposition ceased where the septum touched the surface 

 of the next preceding one, but the margins, where the septa were 

 not in contact, are entire. The margins thus form a sort of frame 

 around the central, elongated, empty space. The siphuncle (Fig. 

 a, G, s^), always near the ventral side of the shell, consists of broad, 

 nummuloid elements ; it is in immediate connexion with the siphuncle 

 of the Nautiloid stage of the Ascoceras shell through a peculiar 

 little tubular duct (Fig. a, dt), which is closed by a calcareous 

 secretion, when decollation has taken place. The form and position 

 of the septa (Fig. a) may be thus described. Starting from the 

 dorsal wall of the shell they first make a strong, inwardly-directed 

 curve, and then sweeping outwards in a wider curve, they closely 

 approach the dorsal side, and finally bend round to the ventral side, 

 thus completely encircling the shell. 



Dr. Lindstrom considers it questionable whether there was any 

 great change in the shape of the animal of Ascoceras, such as the 

 altered form of the septa in the Ascoceras stage would lead one 

 to suppose. That there was some change, at least in volume, he 

 naturally infers, because it seems evident that the shell was, as it 

 were, moulded upon the body of the animal. But he finds, in several 

 instances, evidence of a curious reversion in the shape of the septa 

 and siphuncle to the Nautiloid stage in the Ascoceras. Figure e 

 exhibits this remarkable modification. Dr. Lindstrom argues from this 

 " reversion of characters " that as the animal could scarcely " twice 

 modify its body," there must actually have been very little change 

 in its structure as it passed from the Nautiloid into the Ascoceras 

 stage. 



Referring to the increase in the size of the body of the animal of 

 Ascoceras, as shown by the inflation of the sac-like part of its shell, 

 Dr. Lindstrom remarks : " An increase in the volume of the body 

 must also have occurred in such genera as Gomphoceras and Poterio- 

 ceras, the latter of which bears no slight resemblance to Ascoceras. 

 "When the shell had been completed, the mollusc drew itself higher 

 up in it and commenced the secretion of the sigmoid septa. Near 

 the ventral side the place of the animal has not been much changed ; 

 at the dorsal, again, it moved more and more upwards." 



A short history of Ascoceras here follows, in which the views of 

 different authors, beginning with Barrande, are set forth and 



1 A fragment of an unknown species, figured by Dr. Lindstrom, has indications 

 of no less than twelve septa. 



2 We observe that Dr. Lindstrom uses the term "nucleus," in the same sense; 

 hut it is scarcely so appropriate. 



