20 MADREPOKAEIA. 



seen, has to be the case when the method of growth fails us as in the instances given above. 

 The objections to this, however, are not only to be found in the variation of the calicles 

 on one and the same specimen, but in the practical difficulty of making exact comparisons. 

 The smaUness of the calicles, and the impossibility of carrying the impressions long enough 

 and entire enough in the mind, prevent the exact comparison which is necessary if the 

 calicles are to be chiefly relied upon. Perhaps when photography is more largely used, and 

 enlarged images of the calicles of every specimen can be easily obtained, and closely com- 

 pared side by side, the characters of the calicles will form a safer guide than the methods of 

 growth. 



In the meantime, however, it may be borne in mind, that the form of the corallum is 

 not altogether independent of the form of the individual calicle, the corallum itself being 

 but an aggregate of individual calicles. 



In view, then, of the unsatisfactory nature of any classification based upon these two chief 

 variations, the oft-repeated question once more arises, Is any classification of the various forms 

 composing a genus into separate clearly defined species possible ? I shall not attempt to give 

 any answer to the (luestion in the abstract, but confine myself to describing the result of my 

 own experience while working over tlie Collection of Corals for this and the two following 

 Catalogues (Astrceopora and Montipora). 



The only specimens which can be claimed with absolute certainty as specifically identical 

 are a few which have in each case been gathered at the same place and time, and resemble one 

 another as closely as if they were two fragments of one and the same stock. Beyond these no 

 certainty exists, and strict regard to the variations of form and structure would compel us to 

 label all the remaining specimens as different species or varieties. Further, I do not remember 

 ever having seen a specimen in other private or public collections which exactly recalled any 

 single specimen in the British Museum. Are all these to be classed as new species ? Such a 

 course is only possible wlien the collection dealt with is very small ; but when the number of 

 specimens is measured by hundreds, one's courage faUs. Hence recovirse is had to a recognised 

 but hardly satisfactory system of grouping : certain striking and conspicuous specimens (or 

 single specimens which have already been described by previous workers) are selected as types, 

 and the remainder are divided, according as, in the opinion of the individual worker, they 

 approach one or the other of these favoured specimens. 



The types are thus in the highest degree arbitrary and accidental, as is also, it must be 

 confessed (though in a less degree), the selection of other specimens to be associated with them. 



In the following Catalogue, the 260 specimens of Turbinarians are grouped round some fifty 

 such types. These types are merely so many different forms assumed by specimens of the 

 genus Turhinaria selected for description. The associated specimens appear to resemble these 

 types more nearly than they do any of the other types ; but that they are specifically identical 

 with the type, or with one another (except in the few cases above referred to, where their 

 relationship is indisputable), is more than I should care positively to assert. All that I can 

 actually afQrm is, that the position assigned to them appears to me to be the best. 



