248 MESSRS. BRADY, PARKER, AND JONES 
from that of the earlier and regular segments. The latter are usually smooth, solid, and 
almost structureless; the former are delicately thin, and beset with minute, pointed rugo- 
sities, which have a glassy, suberystalline appearance under a high magnifying-power. 
The difference is probably due to increased or diminished rapidity of deposition under the 
altered relations of the terminal sarcode lobe. | 
The nature of the irregular chambers is best explained by its analogy to conditions occa- 
sionally presented by other types. In luxuriant specimens of many genera, Textularia 
for instance, it is not uncommon to find a chamber abnormally large and unsegmented 
completing the shell—that is to say, sarcode enough for two or three segments of the 
normal size closed in, in bulk as it were, without subdivision. This is no evidence of 
enfeebled vital power as has been suggested *, probably the contrary; for in the localities 
where the tubulose varieties most abound, the Polymorphine, in common with other 
Foraminifera, are remarkable for their fine proportions and apparently vigorous growth. 
Distribution.— Polymorphina Orbignii is widely distributed both geologically and geo- 
graphically. In one or other of its various modifications it has been noticed in the Chalk 
of Bohemia and the South of England, and the Chalk-marl of Kent and Central Europe, 
in the Tertiaries of the South-west of Russia, the Septaria-clays of Northern and Central 
Germany, the Miocene deposits of the Vienna Basin, the Plioeene clays of North Italy, 
and the Crag of our Eastern Counties. In the living state it is found wherever other 
members of the genus are abundant. We have record of its occurrence in the English 
Channel (55-60 fathoms), in the Mediterranean, on the shores of Norway, in the Arctie 
Sea, and at almost every part of our own coast. It is one of the three Polymorphine 
which were obtained by Dr. Sars from a depth of 300 fathoms in the North Atlantic. 
Subgenus DIMORPHINA, D'Orbigny. 
Orthoceratium (in part), Soldani, 1780. ; 
Dimorphina, D’Orbigny, 1826; Sander-Rang, 1829; Menke, 1830; Reuss, 1845; Parker, Jones, and 
Brady, 1865; Schwager, 1866; Karrer, 1868; Von Schlicht, 1869. 
General Characters.—Shell free, inequilateral; commencing growth in alternating 
(obscurely triserial) segments, as in the typical Polymorphine, but becoming uniserial 
after a certain number of chambers are formed. Orifice, at the summit of the terminal 
chamber. 
In the subgenus Dimorphina have been included those few members of the group 
which commence growth regularly on the Polymorphine plan, but subsequently take on 
a Nodosarian arrangement of the chambers. As previously set forth, their separation 
cannot be regarded as absolute or very well-defined, but still sufficiently so to serve a 
practically useful purpose. We have, however, retained in the typical division of the 
group those specimens (like Polymorphina nodosaria, Reuss) whose chambers are oblique 
and alternating, and whose uniserial condition depends on an attenuated habit of growth, 
— à or denis d nte ihe usual overlap of the pense. These ee FRA 
* Dr. Alcock, op. «it. D ; x 
