DISTEIBUTION OF CEUSTACEA. 275 



the Mesozoic formations. In North America they have been traced 

 back to the Devonian period (Palffiopaliemon ; — Macrura), but in 

 tlie European deposits they are not known before the Carbonifer- 

 ous age (Brachypyge ; — Braohyura ?). Whether the astaciform crus- 

 tacean found in the mountain limestone of Ireland, and described as 

 Astacus Philippi, was of a fresh- water habit, may still be considered 

 as more than doubtful. The same doubt extends to the various As- 

 tacomorpha of the Middle Mesozoic period — Eryma, Pseudastacus — 

 and not until we reach the chalk of Westphalia do we meet with 

 any undoubted remains of the genus Astacus itself. But even here 

 ' the deposit in which the remains are imbedded is of a marine facies, 

 and seems to argue that up to this time the crayfishes were by 

 nature inhabitants of salt water. Too much weight cannot, how- 

 ever, be attached to the negative evidence ajiorded by the absence 

 of known fresh-water forms of this group, inasmuch as the chances 

 for their preservation in deposits of this kind among the older 

 rocks is very slim. At the same time, it is not a little singular that 

 the extensive deposits of this nature of Wealden age should be en- 

 tirely barren of their remains. In the Tertiary deposits indispu- 

 tably fresh-water forms are met with, and it is not unlikely that the 

 full diEEerentiation of these types from those of a marine habit was 

 effected somewhere about the close of the Mesozoic era. The 

 lobster or homarine type has probably its oldest representative in 

 the genus Hoploparia, which occurs in the Cretaceous and older 

 Tertiary deposits. Of the modern genera of prawns, Penseus ap- 

 pears to extend back to the Lias, and Palsemon to the Tertiary 

 period. 



The most important order of Cnistacea, considered from the 

 geological standpoint, is that of tlie Trilobita, which, apart from 

 the simple fact of numerical development, acquires special signifi- 

 cance from the circumstance of its repi-esenting the earliest animal 

 group which attained to any prominence in geological history, al- 

 though in point of actual appearance it would seem to have been 

 preceded by the Brachiopoda and Annelida. Thus, from the basal 

 portion of the St. David's beds of South Wales, which rcTJresent 

 the oldest or very nearly the oldest of the fossiliferous rocks that 

 have been thus far discovered, no trilobites are known, and it is 

 not until a full thousand feet in the same series of deposits is 

 passed that their remains are first met with. The number of 



