322 Viscount d'ARCHiAc and M. de Verneuil 



liarises the Devonian period in the west of England, Belgium, the Eifel and the right 

 bank of the Rhine, has also been noticed in the states of Tennessee* and Ohio, 



If we reflect for a moment upon the four principal divisions of the great family 

 of the Terebratulce , that is, the Producti, the Orthides, the Spiriferi and the Terebra- 

 tula proper, all the others being capable of combining with these, we shall perceive 

 that their proportions and their mode of distribution in the three systems being 

 known, one might, by deduction from such knowledge, judge a priori what would 

 be their relative development in succeeding periods. Thus the Producti, which are 

 so suddenly and copiously displayed in the Carboniferous system, would not, it 

 might be inferred, at once disappear in the next overlying deposits, nor survive 

 long after the exceptional circumstances which had caused so rapid an extension ; 

 and, in fact, they disappear above the zechstein and the contemporaneous beds both 

 in Western and Eastern Europe. The Orthides, although decreasing upwards, had 

 still sufficient representatives in the superior system to survive the epoch of the 

 coal-measures ; and one species, 0. Laspii, Buch, lived during the epoch of the 

 zechstein, being found in that formation at Ropsen, near Gera. The Spiriferi, on 

 the contrary, increasing slowly upwards, ought to have survived both the Producti 

 and the Orthides, and they are found even to the upper parts of the lias. Lastly, 

 the Terehratulae, which maintained a nearly equal proportion during the whole of 

 the great series of Palaeozoic rocks, uninfluenced by the changes which took place 

 in the ancient seas, ought to have long outlived all the other genera ; and we observe, 

 in fact, that they have so resisted all the revolutions of which our globe has been 

 the theatre, that they abounded in profusion at all periods, and that they are 

 numerous in the present seas : but by a singular anomaly we do not know any 

 species of Terebratula which lived during the three Palaeozoic epochs. 



It has been seen by the details which we have given of the species common 

 to two systems, that those which have been found most extensively in a vertical 

 direction, or which have the greatest range in the successive deposits, are generally 

 those which have the most extended geographical distribution, and have been met 

 with at the greatest number of places and at very considerable distances ; and 

 indeed it may easily be conceived that the species whose organization enabled 

 them to live through several consecutive periods, would at the same time be those 

 which could exist under the most different external conditions. 



A last remarkable result of the inverse distribution of the species of each genus 

 of Brachiopoda in the three systems, as shown in the foregoing Table, is, that the 

 total of the species in each of them is but shghtly different, and that the number 



* Dr. Troost, Organic Remains, Valley of the Mississippi, Trans. Geol. Soc. Pennsylvania, vol. i. 

 p. 249. 



