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 Palseozoic Era, when two or three species of 

 Leptcena were unexpectedly discovered in the secondary 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 

 lanatomically investigated. Thus, by analogy, we are gradually led, 

 step by step, to reconstruct in our 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 Prodiicta 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 Chonetes comoides several 

 remarkable exteriors and interiors, which, I believe, along with Messrs. Salter, 

 Woodward, and King, to be referable to a single species (PL 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 all 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 



