ae 
SPIRIFER. PENTAMERUS. 391 
peculiar forms render them easily recognisable. Among 
these, the Spirifers are the most interesting, on account of 
their spiral calcareous processes, which in the recent state 
supported the ciliated brachia, being often preserved. A 
specimen, in which part of the upper valve of the shell has 
been removed, and one of the spires exposed, is figured 
Lngn. 126, fig. 3. (Wond. pp. 735, 736).* 
All these genera are extinct; they prevail in the oldest 
fossiliferous rocks, and gradually disappear as we ascend to 
the newer formations; the last trace of their existence is in 
the Lias, in which one species has been found. But the 
Terebratule abound in the Lias, Oolite, Chalk, &., occur 
in the tertiary formations, and several living species inhabit 
the seas around Australia and New Zealand. (See ante p. 390.) 
RHYNCHONELLA, Fischer. The “plaited” Terebratule dif- 
fer from the typical species (e. g. T. australis, caput-serpentis, 
vitrea, &c.) more than even the Spirifers differ, and must be 
regarded as forming a distinct family, RuyNcHONELLIDZ, 
which will include Pentamerus. The shell is not punctate ; 
the arms are spiral, supported only at their origins by 
shelly processes ; the larger valve is beaked acutely, and has 
a notch within the beak through which the pedicle passes; 
sometimes the notch is converted into a foramen, by two 
little plates, (deltidium,) as in Terebratula. The form of 
the Rhynchonelle is tetrahedral. Lign. 125. 
Pentamerus, Ly. p. 352.—With the Spirifers, and other 
Brachiopoda of the Silurian System, some bivalves which, 
in their general figure, resemble certain species of Terebra- 
_ tule, frequently occur. These shells differ in their internal 
structure from all other genera, in having a septum, or 
plate, by which their cavity is divided into four chambers ; 
and in one valve the septum itself contains a cell, thus 
making five chambers, whence the name Pentamerus (five- 
* See a Memoir on the Anatomy of the Brachiopoda, by Professor 
Owen. Zoological Trans. vol. i. p. 145, et seq. 
