744 
THE VOYAGE OF H.M.S. CHALLENGER. 
The first available illustrations of this species ( Operculina complanata), namely, the 
figures given by d’Orbigny with the “ Tableau Methodique,” and his Model, No. 80, 
represent a shell about a quarter of an inch in diameter, of the thin complanate form, 
umbonate at the centre, and with smooth exterior ; composed of about three rapidly 
widening convolutions, the segments of which are extremely numerous and much curved. 
These, therefore, may be regarded as the typical characters of the test. 
In a subsequent description of the genus Operculina by the same author, 1 it is stated 
that the spire is non-embracing ; but this is by no means correct, for even in the present 
species, which often presents an almost completely evolute appearance, the earlier convolu- 
tions are always more or less embracing, and sometimes the alar extensions of the margins 
of the segments are sufficient to create a very marked thickening of the central portions 
of the test. The number of segments in each circuit of the spire differs considerably in dif- 
ferent individuals, and has been recognised by some authors as a basis of “specific” grouping. 
The condition of the exterior of the test with reference to superficial ornament is also 
a point of extreme variability. In any large collection of specimens there are always a 
number that are smooth externally like the d’Orbignian type (PI. CXII. figs. 3, 4 ) ; but 
commonly by far the larger proportion of the shells present a surface-decoration of raised 
granules or tubercles. These are sometimes minute, and scattered over the surface of the 
segments (fig. 10), sometimes in rows along the septal lines, forming a sort of sutural 
limbation (figs. 6, 7), and sometimes of larger size, and developed chiefly near the centre 
of the shell on both faces (fig. 9). More rarely the sutures are marked by bright spots of 
clear shell- substance, which appear like punctations, but are in reality level with the 
surface, neither elevated nor depressed. 
It has been the custom to distinguish the granulose forms specifically from those 
without' surface-markings. It may possibly be convenient to preserve some distinction, but 
it can only be an artificial one ; . for the smooth, the partially ornamented, the granulate, 
and the more strongly papillate varieties form collectively a single continuous series, which 
it is impossible to divide, except by comparative characters of no real zoological value. 
The list of synonyms is probably far from complete, but the drawings of specimens 
referred to the present genus by palaeontological writers are in many cases too small 
and ill-defined to be identified with any certainty. 
Operculina complanata and its granulose modifications are abundant in the Red Sea 
and in the tropical and subtropical regions of the North and South Pacific, at depths rang- 
ing from the littoral zone to 30 or 40 fathoms ; and less common down to 420 fathoms, 
which is the greatest depth at which they have been observed. I can find no record of 
their occurrence in any part of the Atlantic. 
They occur in the Maestricht Chalk, and are plentiful in the Nummulitic limestones 
of Central Europe ; they are also occasionally, though not so frequently, met with in later 
1 For. Foss. Vien., p. 117. 
