‘CLASS II BRACHIOPODA 293 
In the earliest shell growth stages Brachiopod shells are invariably smooth, 
and may remain so throughout life, but the greater number develop radial 
striae, ribs, or undulations, and these are usually crossed by concentric growth 
lines, or lamellae, which are sometimes of great width, or may be extended into 
spines. There may be more or less long tubular spines scattered over one or 
both valves, or sometimes restricted to a single row along the cardinal line. 
Under the term Loricatae, Leopold von Buch included all Brachiopods in which 
the radial folds, or costae, are arranged in regular succession in such manner 
that elevated ridges at the anterior margin of one valve coincide with the 
indentations of the other. In the Biplicatae, a median fold or sinus is 
bordered on either side by a broad fold. In the Cinctae, the plications of the 
two valves meet at the anterior margin in such manner as to form a straight 
instead of a crenulated line, as in the Biplicatae. In young specimens the 
ribs and folds are less prominent and numerous than at maturity. In very old 
or senile individuals the shell is usually thickened and obese, and the growth 
lines are much crowded anteriorly. At this stage old specific characters are 
seen to disappear, and at the same time new ones may be introduced. 
Shell structure.—The test of Brachiopods is composed of laminae of various 
structure and composition, but differs considerably from that of Molluscs. 
The shell may 
be wholly cal- 
careous or alter- 
nately caleareous 
and corneous. 
When entirely 
calcareous the 
laminae are 
never more than 
three in number : 
an inner thick 
prismatic layer, 
an intermediate 

Fic. 490. 
laminar layer ’ A, Punctate exterior of a Terebratuloid shell, slightly 
and an outer enlarged. B, Vertical section of shell of Magellania 
: Prismatic fibrous struc- flavescens, showing distally enlarged tubules, 100/5. 
epidermal film. tureofshell of Rhynchonella C, Inner surface of Magellania shell showing ends of 
: , (Hemithyris) psittacea, 109/; tubules and oblique calcareous prisms, 100/; (after 
The inner lay er (after Carpenter). ‘ Carpenter), 
is made up of 
flattened prisms of calcite arranged parallel to one another with great 
regularity, and forming an acute angle with the surface of the shell (Fig. 
489). In the Thecidiidae these fibrous prisms are so intimately united with 
one another that the shell substance appears almost homogeneous. Very 
often the fibrous layer is perforated by a series of minute canals which pass 
from one surface of the valve to the other in a more or less vertical direction, 
and are somewhat dilated externally. These canals contain tubuli, or certain 
prolongations derived from the mantle, but never communicate with the 
exterior, owing to the fact that the laminar layer of the shell is always 
covered with a chitinous epidermis (periostracum). With the aid of a 
magnifier the openings of these canals are visible in fossil forms, and they 
may be also seen in recent specimens after the epidermis has been removed 
by an application of caustic potash (Fig. 490). According to the presence 
Fic. 489. 
