190 



Devonian rocks and in those of the Carboniferous, Liassic, 

 Oolitic, Purheck, and Wealdeu. Nevertheless he is of opinion 

 that " Although occurring so constantly in the different geoloical 

 periods from the Devonian to the Wealden, and again in recent 

 marine and fresh waters, yet it is in the Triassic deposits of 

 England and the Continent, and in the plant-bearing beds of 

 Virginia and Central India, that this little bivalved Entomostracan 

 appears to be pre-eminently abundant, so as to serve probably 

 as a faithful index of a peculiar geological horizon." 



To the Estheria-bed, succeed about four feet of Clayey Shales, 

 which are followed by the bed known as the Monotis-bed. This 

 is one of the most remarkable in the district for the number of 

 shells it contains of this species, in a beautiful state of preser- 

 vation, with an occasional Modiola minima intermixed. The 

 prevailing fossil on the under side of the slabs on which Mon&tis 

 is found, is a Myacites, which much resembles, if it be not 

 identical with Myacites liassinus of Quenstedt, and is interesting 

 as being the first of that genus and family subsequently so 

 extensively developed throughout the Jurassic period. Monotis 

 seems properly to be considered a sub-genus ofdvicula, differing 

 no more from the species with prolonged bases or hinge-lines, 

 which subsequently take its place, than do the Meleayrince from 

 the Avicula of the present time. Considered then as an Avicula 

 it is a form which is known to us in the " Caradoc" and probably 

 lower still. 



From a consideration of the foregoing circumstances it will 

 be seen that in connexion with mineralogical characters entirely 

 liassic, we have a zone of life which as certainly appears to have 

 its affinites with the preceding period. How far it may be wholly 

 assignable to the " Trias " as has been done by some competent 

 geologists, may perhaps still admit of question. Many regard 

 the entire zone, comprising on the Continent of Europe a 

 succession of beds not less than 200 feet in thickness, as repre- 

 senting a truly transitionary period, and as such entitled to a 

 separate and distinct rank, having analogies with both the 

 contiguous formations, yet strictly assignable to neither. Here 

 at any rate is a field for study and observation which can only be 

 successfully worked out by a careful comparison of the series of 

 beds as displayed in our own county at Westbury, Wainlode, 

 Aust, &c., with the same beds at Binton and other localities in 

 Warwickshire, at Penarth near Cardiff, at Axmouth in Devon- 

 shire, and wherever else they may be exposed in the British 

 Islands ; and these again must be correlated with those on the 

 Continent of Europe, where a more extensive development 

 illustrated by a far larger series of fossil remains, will no doubt 

 eventually enable geologists to determine with exactitude and 

 correctness, the true value of the beds in the geological scale. 



With reference to the upper beds of the " New Red," and in 



