vi INTRODUCTION. 



It will probably be thought by many that in the descriptions of the species, and, 

 perhaps also, in the generic diagnostic plu'ases, I have entered into needless minutiae 

 of detail ; and that it would have been as well if I had limited myself to those characters 

 which are fomid sufficient in the descriptions of recent forms. But when it is recollected 

 that most of the specimens occm- in a more or less fragmentary condition, and that 

 it often happens that a small portion only of an individual is preserved, it will be obvious 

 that unless the description includes every part which remains (and this is often too little 

 for accurate or certain determination), there would be no means of ascertaining the 

 identity of newly-found si)ecimcns with the tyj)e of the previous description. I have, 

 therefore, notwithstanding this apparent tediousness, given the fullest detail in my power 

 of the difl'erent parts Avhich still remain, so that, as far as possible, there may be some 

 grounds for future satisfactory comparison. In the description of recent forms this 

 minuteness of detail is obviously unnecessary, as it may be fairly expected that the object 

 to be coni])ared will he nearly or quite perfect and unmutilated. With all possible care, 

 however, it must still often happen that characters of great importance both in distin- 

 guishing species, and (which is of even more frequent occurrence, and moi'c important with 

 reference to the higher views of zoological science), in ascertaining the relations of genera, 

 are absolutely wanting, or so defective as to be scarcely available. 



Amongst the remains of an earlier period which are found in the Suffolk Crag, 

 are several species of the Crustacea peculiar to the London Clay, and I may particularize 

 two species of Xanf/iopsis, Xa/U/wlifes Bowerbankii, Dromilites Lamarcku, and two 

 species of Macrura ; all the spccuncns which have come under my observation have been 

 much rolled and worn. 



It is unnecessary to urge upon those who are accustomed to the study of Crustacea, 

 the necessity of great caution lest the same species at different ages should be described as 

 distinct ; and if this be the case with respect to these animals in the perfect condition 

 in which the recent species are obtained, and where all the organs are consequently 

 submitted to examination, it is far more so when fragments only are obtainable, or, 

 at best, where many of the organs most important in the discrimination of species are lost. 

 Hence, we may presume, has arisen the mistake which has become public by the printed 

 statement of Professor Monis, in the preface to his admirable ' Catalogue,' that in the 

 collection of Mr. Bowcrbank alone, there are not less than thirty species of Cmstacea 

 from the London Clay. I have most carefidly examined that admirable collection, the 

 equally numerous one of Mr. Wetherell, that of the British Museum, of the College of 

 Surgeons, and of the Woodwardian Museum at Cambridge ; and these, with the addition 

 of my own and one or two other small collections, have afforded in the whole, not more 

 than nineteen species. Of these ten belong to the Brachym'a, three to the Anonmra, and 

 six to the Macrm-a. 



Of these species, the greater uumljer of the Brachym-a are found in the greatest 



