PRELIMINARY REMARKS. 5 



than a fifth of the length of the shell. We have in vain attempted to find out some external 

 character by which we might discover if the species had a long or short loop ; it had been 

 imagined that those with a long loop were flatter, depressed in the smaller valve, and with 

 lateral ridges of the beak strongly defined ; but this character falls to the ground when we 

 place in comparison such shells as Ter.perovalis and punctata, that have a short loop, with 

 Ter. ornithocephala, digona, and others with a long one, all possessing a similar convexity of 

 valves and other external characters. I have lately observed that the crura differs slightly 

 in those specimens with a short or long process, as may be seen from the figures illustrating 

 the interiors of Ter. Cornuta, resupinata, ornithocephala, &c, on one side, and those of 

 Ter. Maxillata, intermedia, spharoidalis, &c, on the other; and it will be perceived that in 

 the elongated looped shells the crural base is much larger, and forms a flat surface, which 

 does not exist in a similar manner in the others. 



The punctures visible in Terebratulse and other genera cannot be used as a general 

 character to distinguish species, as the results of microscopical examination show us that 

 their size and form varies on the same shell, according to the portion placed under exa- 

 mination ; thus on one part we find the punctures widely separated becoming closer and 

 larger towards the edge until they assume a completely different appearance ; and 

 although some species have a marked difference in this respect, others present no dis- 

 tinctions. Professor King, in his valuable ' Monograph on Permian Fossils,' goes too 

 far, in our opinion, while stating that he believes few Brachiopoda unpunctuated, as very 

 many shells in this class show no traces of punctures ; but at the same time we believe 

 that many species considered unpunctuated are really so. It had been supposed, till very 

 lately, that the Liasic species of spirifers were characterised and distinguished by being 

 punctuated, while other spirifers were not so ; this opinion, must however, be abandoned, 

 from the fact observed by Professor King, that many carboniferous spirifers, very different 

 in shape and character, were likewise so, which has been also confirmed by M. De Koninck 

 and others. Before entering into the subject of this Monograph, we think it necessary to 

 state, that the confusion and contradictions we have found in authors were so great that 

 it has in many cases been no easy matter to find out the original types. Many of 

 Sowerby's and Lamarck's species, as well as those of other authors, have been singularly 

 misrepresented, and in fact have been but little understood by the generality of Palaeontolo- 

 gists and Geologists, who usually do not attach much importance to these determinations. 



While reviewing the Lamarckian species of Terebratulse, I was surprised to find in that 

 celebrated collection many shells quite differing in species from those intended by the 

 author of the 'Min. Con.;' and these errors seem so current abroad, that even the most 

 distinguished Palaeontologists, such as Von Buch, Bronn, D'Orbigny, and others, have 

 placed in their catalogues, as Sowerby's species, shells, differing completely, as will be 

 alluded to during the course of this work. Many of these mistakes are excusable, and 

 might have been expected, from the unfortunate foreshortened position in which that 

 author represented some of his shells. Through the kindness of Mr. J. de C. Sowerby, who 



