208 
THE VOYAGE OF H.M.S. CHALLENGER. 
material furnishes almost every variety, from the longest to the widest. The fact 
however remains, these exceptional cases notwithstanding, that the home of Peneroplis 
is at depths of less than 30 fathoms. 
The geological range of the species is limited to the Tertiary epoch. It is to be met 
with from time to time in shallow -water deposits from the Eocene of the neighbourhood 
of Paris to the Crag of our eastern counties, or probably to an even later geological period. 
Orbiculina , Lamarck. 
Nautilus, pars, Fichtel and Moll [1803], 
Archaias, Helenis, Ilotus, Montfort [1808]. 
Orbiculina, Lamarck [1816], Deslongchamps, Defrance, Blainville, d’Orbigny, Bronn, Williamson, 
Carpenter, Parker and Jones, Morris and Quekett, Reuss, Carter, Pourtales. 
The genus Orbiculina is typified by a planospiral, porcellanous shell, the early 
convolutions of which are more or less equitant or embracing, and the constituent 
chambers narrow and regularly subdivided into chamberlets. 
The typical nautiloid condition is liable to modification in two directions. Sometimes 
the successive segments do not increase very rapidly in size, and, instead of continuing the 
original spiral arrangement, are superimposed in a straight or curved linear series, so as to 
form a crosier-shaped test (PI. XIV. fig. 4). On the other hand, the successive chambers 
may increase so rapidly in length and curvature that after a time they completely en- 
circle the shell, which subsequently assumes a discoidal form and an annular mode of 
growth (figs. 8, 9). 
The texture of the shell is homogeneous and imperforate, its general appearance white 
and polished. The surface of the test, both external and internal, is either quite 
smooth, or more frequently marked by minute pits or depressions (PI. XIV. fig. 13), which 
at first sight may easily be mistaken for perforations. The true nature of these mark- 
ings was long ago pointed out by Carpenter (Phil. Trans., 1856, p. 551), who dispelled 
the idea that they arose from anything more than minute superficial punctations, as had 
been previously supposed. 
Orbiculina in all its modifications differs from Peneroplis in having subdivided cham- 
bers. The discoidal forms are distinguished from Orbitolites, which they otherwise 
resemble, by the convexity of the umbilical region of the shell, resulting from the 
investing character of the early convolutions. 
The varieties of Orbiculina are all referrible to a single species, the details of the 
geographical and geological distribution of which are given in a subsequent paragraph. 
The history of the genus has already been written with a completeness that leaves 
little room for additions, and the reader may be referred to the memoirs published by 
