THE DECAPODA 307 



have indeed been described as belonging to this group, but in no 

 case is enough known of their characters to enable more to be said 

 than that they agree with the " caridoid " groups of the Malacostraca 

 in the possession of a carapace and of a tail-fan. 



In the Mesozoic rocks many undoubted Decapods occur, includ- 

 ing representatives of all the chief groups now living. Many 

 genera of Penaeidea are found from the Jurassic, perhaps from 

 the Triassic period onwards, some of the earliest even resembling 

 closely the existing genus Penaeus, to which they have been referred. 

 Aeger, from Triassic and Jurassic rocks, presents characters which 

 suggest an affinity with the Stenopidea. True Caridea appear later, 

 in the Upper Jurassic, some at least presenting primitive characters 

 in the retention of exopodites on the ambulatory limbs. Fresh- 

 water Caridea of doubtful affinities occur in the Miocene. The 

 Eryonidea are especially interesting since the few existing deep- 



FJO. 184. 



Larger cheliped of Ocypoda macrocera, from the inner side, showing the stridulating 

 mechanism, a, file-like series of ridges on propodite ; h, ridge or scraper on ischiopodite 

 against which the ridges of the propodite can be rubbed when the limb is flexed. (From 

 Alcock, Naturalist in Indian Seas.) 



sea forms appear to be only the surviving remnants of what was in 

 the Mesozoic period a dominant group. The genus Eryori (Fig. 185) 

 appears in the Trias and persists until the earlier Cretaceous. The 

 Glyphaeidae, a wholly extinct group having much the same range 

 in time as have the fossil Eryonidae, have been supposed to stand in 

 the direct line of descent of the Scyllaridea. True Scyllaridea occur 

 probably in the Jurassic, certainly in the Cretaceous period. The 

 existing genus Linuparus, or a very close ally, dates back to the 

 upper Chalk. Astacura are known from Jurassic and later deposits 

 in considerable numbers. Eryma, from the Lias, and Hoploparia 

 (Cretaceous and Tertiary) are well-known forms. 



The Anomura are almost unknown as fossils, except for some 

 Thalassinidea referred to the existing genus Callianassa occurring 

 from the Upper Jurassic onwards. The Brachyura, on the other 

 hand, are well represented. The earliest forms present characters 

 of the Dromiacea, and are referred, for the most part, to the extinct 

 family Prosoponidae, which Bouvier has shown to have close rela- 

 tions with the most primitive of existing Brachyura, the Homolo- 



