PRELIMINARY REMARKS. 7 



from the interminable list of perplexing varieties, impossible to be remembered. Several of 

 these M. Deslongchamps admits, such as Ter. Trlguetra and Ornithocephala, themselves 

 only varieties of age and shape of the same species. We therefore think it preferable, for 

 the sake of convenience., to admit artificially more species and less variety ; and, in order 

 to incorporate certain smaller variations, the description of a species should be so framed as 

 to embrace the general idea of the thus limited form rather than the account of a single 

 specimen. 



Local causes and malformation, caused by pressure or fracture during the different 

 stages of growth, are not to be wondered at in a class of shells destined in general to 

 sedentary life, living and dying on the same spot attached to rocks, or in circumstances 

 where there existed a want of room for their complete development. These malformations, 

 alluded to by M. Deslongchamps, are much more common than generally imagined, 

 and have often been made into distinct species by different authors, from the inspection of 

 a single specimen. It is likewise certain, notwithstanding what may have been said to 

 the contrary by some more learned Palaeontologists, that the limits in vertical range of 

 some species have extended to more than one group of strata, although species commonly 

 are characteristic, and restricted to narrow bounds. We must also allude to some technical 

 denominations made use of to denote the different parts of a shell ; thus, for a considerable 

 period, and in many important works, the term dorsal has been applied to the larger or 

 perforated valve, and that of ventral to the smaller one. I do not wish here to contest 

 the observations which induced the celebrated anatomist, Professor Owen, to reverse the 

 denomination of the valves from the relative disposition of the animal to the valves, which 

 in some genera, such as Orbicula, would be the reverse of that in Terebratula. Because of 

 the immense confusion such a change would unavoidably create in works already pub- 

 lished, we have determined to banish completely the terms dorsal and ventral from our 

 descriptions, and to adopt other terms, also in use, to distinguish the same parts : thus we 

 will use indiscriminately the words perforated, rostral, or large valve, for the one considered 

 by De Buch and others a dorsal, the ventral of Professor Owen, and that of imperforated, 

 upper or smaller, for the lesser valve. 



So variable are Brachiopodes in shape, size, gibbosity, &c, that we cannot employ 

 angular measurements proposed by Von Buch, as those characters vary in every specimen ; 

 nor do we attach much importance to the dimensions we give to each species. In general 

 we have taken those of the best developed specimen which has come under our notice. 

 Nor are the number of plaits more constant. We find the same species, especially among 

 the Bhjnchonellas, have one, two, three, four, or more plaits on the mesial fold ; thus most 

 species, smooth in the young, are plaited or otherwise ornamented at a more advanced 

 period of growth; it is therefore, in most cases, impossible to determine species from 

 young shells. 



In concluding these few preliminary remarks, it is necessary to state, that merely the 

 synonyms and references likely to prove useful are inserted, as it would have been im- 



