24 FOSSIL MALACOSTRACOUS CRUSTACEA. 



A beautiful specimen in the Museum of Practical Geology, a mutilated one in the 

 British Museum, both from the Gault at Folkestone, and several in Mr. Carter's collection 

 from the upper Green sand at Cambridge, have formed the basis of the above description ; 

 unfortunately, the carapace alone remains in all cases, without a vestige of limbs or of any 

 other organs. 



Order— MACBUBA. 

 Family— ASTACIDjE, 



Genus — Hoploparia, M'Coy. 

 The generic characters will be found in the first part of this monograph, p. 36. 



Additional Observations. 



The extent, both in time and space, in which the different species of this genus occur 

 in the deposits of seas of very remote epochs, is deserving of particular remark. In the 

 London Clay two well-marked species have been found, and have been already described 

 in this work ; and as low as the Greensand at Lyme Regis the existence of a species was 

 long since made known by the late Mr. George Sowerby. I have now to describe no less 

 than two more from the Gault, besides two or three others found in different beds 

 of Greensand in various localities. The generic characters first seized upon by Professor 

 M'Coy cannot be mistaken, and they are equally appreciable in whatever strata the dif- 

 ferent species may occur. It would, in fact, be difficult to name a genus, either fossil or 

 recent, of which the characters are more definite, and the different species of which are 

 more clearly demonstrable. It is therefore the more remarkable that, with the immense 

 interposition of the whole Chalk formation, the genus so distinctly marked as belonging to 

 the Greensand and Gault is, as it were, reproduced in the London Clay, under only slight, 

 although definite, specific modifications. This would surely indicate that the physical con- 

 ditions necessary for the propagation and maintenance of this particular form of macrurous 

 Crustacea existing in that early period when the older members of the cretaceous group 

 were formed, should, after the incalculable interspace occupied by the great Chalk deposit, 

 during which period we have no trace of the genus, have again prevailed, and favoured or 

 permitted its development. How these facts are to be accounted for upon the hypothesis 

 of " selection," or of the gradual transformation or development of species, appears to me 

 inexplicable. Even were we to allow that the species in each of the different formations 

 may possibly have resulted from variation of one original form, an opinion utterly at 



