60 INTRODUCTION. 



species do not possess the character assigned by the author to his genus, viz.: — A great 

 number possessed spiral calcified processes for the support of the arms, similar to 

 those seen in Tiimida and Herculea, and were not consequently free, such as pimm. 

 Sow., Nitida, Hall, and a number of the species taken from M. Barrande, and which 

 partly belong to the genus Athpis, M'Coy, {Spiri(/era of D'Orbigny,) while a few, 

 such as prunum, from possessing vertical spires, require to be classed with the true 

 Atrypce of Dalman, or should, according to the French author's views, have been admitted 

 into his genus Spirigerina. M. d'Orbigny's genus Atrypa is, therefore, made up 

 of species belonging to the genus BhyncJtonella (which he admits), as well as of others 

 partaking of the character of his Spiriyera and Spirigerina, &c. Numerous examples 

 could be adduced to show^ the difficulty at present existing in the classing of many species 

 into their true genus or place from mere external appearances, for, if we are to characterise 

 a group by the presence of free extensile arms simply fixed at their origin to two slender 

 curved processes, departing from the hinge-plates, and another by the arms being sup- 

 ported throughout in a peculiar manner by spiral lamellae, forming vertical and horizontal 

 cones, it is evident that species with the first character cannot take place along with 

 those of the second ; and experience has proved that often two forms, somewhat similar 

 exteriorly, may have been inhabited by a very different animal, the one with free, probably 

 extensile arms, the second in which that appendage was unextensile, and entirely supported 

 by a largely-developed calcified process, &c. The only way, therefore, to be able with time 

 to arrive at a proper and natural catalogue of all the species, is to examine and develop the 

 internal dispositions of those forms as yet uninvestigated, and to gradually and carefully 

 illustrate their characters. A great difference exists likewise on the probable periods of the 

 first apparition and extinction of the different genera, and to this point geologists as well 

 as Palaeontologists justly attach great importance ; some few genera, viz., Lingula, 

 Discina, Crania, and Bhynchonella, appear to have traversed the whole geological vertical 

 range; they appear in the older Silurian deposits, and with similar or but slight modifications 

 in character, are still represented in our seas by a limited number of species. The Tere- 

 bratulcB with short loops appear, from the present state of our information, to have first 

 appeared during the Devonian era, since no example has as yet been pointed out in older 

 rocks ; but it is well to observe, that the genus Terebratula seems to have been of very 

 rare occurrence in the Palaeozoic period, and to have become truly developed only from the 

 epoch of the Oolites to the present day. TerehratulcB with long loops {JValdheimia), as well 

 as the other sections in the Family Terebratulid^, appear, during the Oolitic period, 

 and upwards ; thus, for example, Waldheimia and Terebratulina are found in the Oolitic 

 Cretaceous, Tertiary, and present epoch, Meyerlia and Argiope only in the last three, 

 while Kraussia, Bouchardia, and Morrisia, are only positively known in the present 

 period. 



Thecidium has not yet been discovered under the Oolites, and there also it seems to 

 have attained its maximum of specific development, although not much less numerous in 



