Dr. F. A. Bather — Notes on Yunnan Cystidea. 71 



were cumbered with a very buoyant shell. Solger 1 concluded from 

 the fact that an injury to the chambers and destruction of several 

 septa made no apparent difference to the animal, which continued 

 its growth without noticeable irregularity, that the Ammonite was 

 benthonic before the accident and could not have been a swimmer. 

 But this may only show that the buoyancy did not depend on the 

 air-chambers only; and "bulk" in the case of a sharp-ventered, 

 thin shell, was not necessary to enable its occupant to swim well 

 and quickly. The damage to the earlier whorls of large and evolute 

 shells at a late stage (large Arietids often have no centre like certain 

 perforated Nautili of the Palaeozoic or Triassic) probably would not 

 have affected the buoyancy of the shell and mode of life of the 

 animal more than the loss of its apex would have affected an 

 Orthoceras. 



The problem of disposing of superfluous calcium salts was 

 probably never a pressing one in Ammonoids, for as in other tubular 

 organisms growth could generally continue indefinitely whether the 

 cone was coiled or straight, and when a large amount of mineral 

 matter went to elaboration of one feature, the equilibrium was 

 generally maintained. The periodical thickenings of ridges and 

 flares, e.g., which are also found in the long-lived genera Phylloceras 

 and Lytoceras, cannot in the writer's opinion be looked upon as such 

 deposits of superfluous calcium-carbonate, after the manner of 

 tabulae, diaphragms, etc., in other tubular organisms, 2 and it must 

 be remembered that very elaborate apertural structures seem to have 

 been resorbed occasionally. A more fundamental problem than the 

 necessity of getting rid of an excess of calcium-salts is presented by 

 Pinacoceras, which represents the extreme in sutural elaboration 

 among Ammonoids, and where the septal edge seems to be com- 

 plicated beyond utility and mechanical requirements, and by the 

 crowding of septa, which also occurs during the Triassic acme of the 

 group. This is probably comparable with the specialization and 

 running to extremes in other directions, e.g., size, ornamentation, or 

 uncoiling, a phenomenon not confined to Ammonoid phylogeny, and 

 generally followed by the extinction of the specialized lineage at or 

 near the height of its career. 



V. — Notes on Yunnan Cystidea. III. Sinocystis compared with 



SIMILAR GENERA. 



By F. A. Bather, D.Sc, F.E.S. 



(Published by permission of the Trustees of the British Museum.) 

 A. — Comparison with Aristocystis, Hippocystis n.g., Calix, 



AND ArCHEGOCYSTIS. 



THE Diploporita, to which Order Sinocystis clearly belongs, have 

 been divided into Families according to the greater or less 

 extension of the subvective system over the theca and the 

 modifications thus induced in the arrangement of the thecal plates. 



1 " Fossilien d. Mungo-Kreide " : Geol. v. Kamerun, ii, p. 216, 1904. 



2 W. D. Lang, "Calcium Carbonate and Evolution in Polyzoa " : GEOL. 

 Mag., Dec. VI, Vol. Ill, No. 620, pp. 74-5, February, 1916. 



