1872.] 11 (Hyatt. 
exaggerate these peculiarities. The geological succession does not 
strictly accord with the manner in which the organization of the 
species arranges them in the series. 
Platypleuroceras brevispina occurs in the lowest bed of the Middle 
Lias, Jamesonibed, and Cycloceras bipunctatus and Actcon in the 
next, or Ibexbed; but Cycloceras Masseanum, instead of following 
these species, as would be naturally expected, is associated with 
Platypleuroceras brevispma. This discrepancy will probably be ulti- 
mately explained, but at present it must be quoted as an exception 
to the usual law of agreement between zoological rank as determ- 
ined by development and geological succession in time. It is only - 
fair, however, to state that the affinities of Cycloceras Masseanum 
were considered very doubtful before its geological position was as- 
certained. 
PLATYPLEUROCERAS. 
Platypleuroceras brevispina. 
Amm. brevispina Sow., Min. Conch., vol. vi, p. 106, pl. 556. 
Amm. natriz (rotundus et oblongus) Quenstedt, Die Ceph., p. 85, 
pl. 4, figs. 16, 17. 
Platypleuroceras lateecosta Hyatt, Bull. Mus. Comp. Zoology, no. v, 
p- 92. 
Cycloceras molare Hyatt, Bull. Mus. Comp. Zoology, no. v, p. 92. 
In this species we find a more singular combination of the charac- 
teristics of Microderoceras and Microceras. ‘The young are for a 
lengthened period, until, in fact, they are over two inches in diame- 
ter, very similar to Microderoceras Birchu. ‘Then the pile begin to 
stretch across the abdomen, forming a whorl closely identical exter- 
nally with that of the adult of Microceras latecosta. ‘The double line 
of tubercles is retained, however, much more persistently, and is 
usually more prominent than in the latter. The septa are very dis- 
tinct at the same age in this species, the sutures being much more 
complicated, with more prominent cells and deeper lobes, and also 
as in Mucroderoceras, very deeply divided by three minor lobes. 
In the adult this contrast is not so great; the lobes and cells appear 
to spread considerably, and become more like those of the adult of 
Microceras latcecosta. 
