396 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. [FoZ.VI. 



Wealden. " In the marine deposits of tlie Cretaceous epoch, however, 

 astacomorphous forms, which are known by the generic names of Roplo- 

 paria and Uno^jloclytia, are abundant. 



" The differences between these two genera, and between both and 

 Uryma, are altogether insignificant from a broad morphological point of 

 view. They appear to me " to be of less importance than those which 

 obtain between the different existing genera of crayfishes. 



'' Hoploparia is found in the London clay. It therefore extends be- 

 yond the bounds of the Mesozoic epoch into the older Tertiary. But 

 when this genus is compared with the existing Homarus and Weplirops^ it 

 is found partly to resemble the one and partly the other. Thus, on one 

 line, the actual series of forms which have succeeded one another from 

 the Liassic epoch to the present day is such as must have existed if the 

 common lobster and the !N"orway lobster are the descendants of Erymoid 

 crustaceans which inhabited the seas of the Liassic epoch. 



" Side by side with Uryma, in the lithographic slates, there is a genus, 

 Pseudastamis, which, as its name implies, has an extraordinarily close re- 

 semblance to the crayfishes of the present day. Indeed there is no 

 point of any importance in which (in the absence of any knowledge of 

 the abdominal appendages in the males) it differs from them. On the 

 other hand, in some features, as in the structure of the carapace, it 

 differs from JEryma, much as the existing crayfishes differ from Neplirops. 

 Thus in the latter part of the Jurassic epoch the Astacine type was already 

 distinct from the Homarine tyi^e, though both were marine ; and, since 

 Eryma begins at least as early as the Middle Lias, it is possible that 

 Pseudastacus goes back as far, and that the common protastacine form is 

 to be sought in the Trias. Pseudastacus is found in the marine cre- 

 taceous rocks of the Lebanon, but has not yet been traced into the 

 Tertiary formations. 



" I am disposed to think that Pseudastacus is comparable to such a 

 form as Astacus nigrescens rather than to any of the Parastacidse, as I 

 doubt the existence of the latter group at any time in northern lati- 

 tudes. 



"In the chalk of Westphalia (also a marine deposit) a single speci- 

 men of another Astacomorph has been discovered, which possesses an 

 especial interest as it is a true Astacus {A. politus von der Marck and 

 Schluter), provided with the characteristic transversely divided telson 

 which is found in the majority of the Potamobiidse. * * * 



"If an Astacomorphous crustacean, having characters intermediate 

 between those of Eryma and those of Pseudastacus, existed in the Jurassic 

 epoch or earlier; if it gradually diverged into Pseudastacine and 

 Erymoid forms ; if these again took on Astacine and Homarine char- 

 acters, and finally ended in the existing Potamobiidse and Homarina, 

 the fossil forms left in the track of this process of evolution would be 

 very much what they actually are. Up to the end of the Mesozoic epoch 

 the only known Potamobiidee are marine animals. And we have already 



