DAVIDSON — PAL^ONTOLOGICAL NOTES ON THE BRACHIOPODA. 103 
All the species at present known are restricted to the limits of a 
portion of the Palaeozoic Period ; but experience has taught us that 
any day may bring forth the discovery of some form in a higher or 
lower stage ; thus, until 1847, the Strophomenidce were considered to 
be limited to the Palaeozoic Era, when two or three species of 
Leptcena were unexpectedly discovered in the secondaiy or Mesozoic 
strata ; and this first discovery led to that of a large number of 
species in the same and other localities. 
As no living representatives of the families exist, the character of 
those portions of the animal can alone be deciphered and described 
which have left their impressions upon the interior surface of the 
shell ; hence the necessity of carefully seeking for these marks with 
great attention, and of comparing them with those observable upon, 
the internal surfaces of species of other families which have been 
anatomically investigated. Thus, by analogy, we are gradually led, 
step by step, to reconstruct in oQr minds the animal which has for count- 
less ages ceased to be represented in the successive series of creations. 
In this paper we shall treat of the Productid^, and we com- 
mence with the external characters. The species are very numerous, 
and among them may be seen some of the largest Brachiopoda at 
present known. The shell is concavo-convex, regular or irregular 
in its growth ; transverse or elongated, more often oval, semi-oval, or 
angular, and generally auriculate. The hinge-line is long and straight, 
with or without teeth and sockets for the articulation of the valves. 
All well-authenticated species of Producta and Aulosteges hitherto 
examined have shown themselves to be edentulous ; but whether such 
character was general to all the species, or only peculiar to a certain 
number, will require further confirmation.* Anyhow (as has been 
* In the tenth volume of the Quarterly Journal of the Geological Society 
(p. 202, pi. VIII. 1853), I described and figured as Ckonetes comoides several 
remarkable exteriors and interiors, which, I believe, along with Messrs. Salter, 
Woodward, and King, to be referable to a single species (PI. IV. fig. 7). The 
sharply defined and well-developed area in each valve, the fissure in the ventral 
one, and the produced cardinal process in the other, as well as the strongly 
articulated hinge, have up to the present time been considered characters peculiar 
to Chonetes, and not to Producta, especially since aU well-authenticated species 
and specimens (hitherto examined) of the last-named genus have proved to be 
edentulous. Small pits, observable at intervals along the cardinal edge in several 
examples which I then or have subsequently examined, seemed also to denote the 
existence of small cardinal spines, similar to those observable in certain species of 
Chonetes ; nor do the muscular impressions in the interior of the ventral valve 
(unfortunately only one specimen is at present known) mihtate against such a 
